JOURNAL OF BACTERIOLOGY, Mar. 1974, p. 1108-1116Copyright 0 1974 American Society for Microbiology

Vol. 117, No. 3Printed in U.S.A.

Amino Acid Pools and Metabolism During the Cell DivisionCycle of Arginine-Grown Candida utilis

P. NURSE AND A. WIEMKEN

Department of General Botany, Swiss Federal Institute of Technology, CH-8006 Zurich, Switzerland, andSchool of Biological Sciences, University of East Anglia, Norwich, United Kingdom

Received for publication 16 October 1973

Synchronous cultures obtained by isopycnic density gradient centrifugationare used to investigate amino acid metabolism during the cell division cycle ofthe food yeast Candida utilis. Isotopic labeling experiments demonstrate that therates of uptake and catabolism of arginine, the sole source of nitrogen, doubleabruptly during the first half of the cycle, while the cells undergo bud expansion.This is accompanied by a doubling in rate of amino acid biosynthesis, and an

accumulation of amino acids. The accumulation probably occurs within thestorage pools of the vacuoles. Amino acids derived from protein degradationcontribute little to this accumulation. For the remainder of the cell cycle, duringcell separation and until the next bud initiation, the rates of uptake andcatabolism of arginine and amino acid biosynthesis remain constant. Despite theabrupt doubling in the rate of formation of amino acid pools, their rate ofutilization for macromolecular synthesis increases steadily throughout the cycle.The significance of this temporal organization of nitrogen source uptake andamino acid metabolism during the cell division cycle is discussed.

The amino acids are by far the largest compo-nent of the soluble low-molecular-weight inter-mediates found in yeast (5), accounting forabout 90% of the carbon extractable from Can-dida utilis by cold trichloracetic acid (6). Theyact as substrates for numerous biosyntheticreactions of intermediary metabolism and mac-romolecular synthesis, and are also importantas effector molecules involved in the regulationof metabolism (22, 32). Because of these diverseand sometimes antagonistic roles, it is impor-tant to understand how the soluble amino acidsare organized both in space within the living celland also in time as the cell progresses throughthe cell division cycle.

In a previous report we discussed the spatialorganization of the soluble amino acids in C.utilis and Saccharomyces cerevisiae (35, 36). Asmall pool tuming over rapidly was recoveredfrom the cytoplasmic compartment, and a largepool turning over slowly was recovered from thevacuoles. These results indicate that a largeproportion of the soluble amino acids is sepa-rated from the main sites of metabolite inter-conversion and macromolecular synthesis, bybeing sequestered within the vacuoles. Thevacuoles were also observed to undergo strikingchanges during the budding cycle of S. cerevis-iae (34). They enlarge and coalesce during budextension whereas later, shortly before new

buds appear, they shrink and fragment to formnumerous small vacuoles. These observationsstrongly suggest that changes occur in thevacuolar pools of amino acids during the bud-ding cycle. The objective of the work describedin this report is to investigate further thechanges in the amino acid pools during the cellcycle of C. utilis.

MATERIALS AND METHODS

Culture conditions. C. utilis NCYC 737 (originallydesignated Torulopsis utilis BP 60) was grown on de-fined media containing salts and trace elements asdescribed by Ferguson and Sims (8) and 0.5 or 1.0%glycerol or glucose as carbon source, and 5 or 10 mMarginine, glycine, potassium nitrate, or ammonia asnitrogen source. Precise growth conditions are de-scribed by Wiemken and Nurse (35).

Fractionation of a yeast cell population. Densitygradients of dextrin (obtained from Fluka, Buchs,Switzerland) were used for isopycnic centrifugation ofyeast cell populations from the exponential phase ofgrowth (31). A 29% (wt/vol) stock solution of dextrinin culture medium was prepared and filtered througha Whatman no. 50 hard filter paper to removeinsoluble material. Using different dilutions of thisstock solution, continuous or stepped gradients wereprepared ranging in density between 1.088 and 1.105g/cm3 at 4 C. The precise range which was used foroptimal separation of the cells varied slightly depend-ing on the culture conditions. All the fractionationprocedures were carried out between 0 and 4 C. The

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AMINO ACIDS DURING C. UTILIS CELL CYCLE

exponentially growing culture that was to be fraction-ated was harvested either by centrifugation afterbeing rapidly chilled on ice, or by filtration onWhatman GF/C glass fiber filters. The pelleted cellswere then resuspended in a small volume of coldculture medium and were loaded on top of a dextrindensity gradient. Up to 1.5 x 1010 cells could beloaded onto a single gradient of 20-ml capacity. Thegradients were centrifuged in a swing-out rotor for 10min at 45,000 x g or for 20 min at 27,000 x g. The cellfractions obtained from the gradients were washedfree of dextrin by centrifugation in cold medium.The washed fractions were examined in a phase-

contrast microscope (x400), and the proportions ofthe various cell types were counted. Two classes of celltypes were routinely used for monitoring the cellcycle, initial budding cells with small buds and latebudding cells with large buds.

Fixation and extraction of separated cells. Thosecell fractions that were to be immediately fixed foranalysis were further washed in cold buffer as con-tained in the culture medium and with distilledwater. These washes do not extract any amino acidsfrom inside the cell. The cells were then disrupted bysuspending in absolute methanol. They were cooled to-15 C, and the methanol was removed by evapora-tion on a vacuum desiccator. The disrupted cells wereresuspended in a small volume of distilled water(approximately 1 ml per 50 mg [dry weight] of cells)which extracted, within 20 min at room temperature,all the soluble amino acids. After centrifugation thesupernatant was decanted and used for analysis of thesoluble amino acids. The sediment was used forprotein determination.Growth and sampling of synchronous cultures.

The bottom ffactions of the gradients, containingpredominantly initial budding cells, were used as theinoculum for synchronous cultures. Cultures wereusually grown for at least 100 min before sampling toinsure steady-state growth. The culture was thenfollowed and sampled for about a further 1.5 celldivision cycles. Progress of the synchronous culturewas monitored by counting the proportions of the celltypes as defined above. Cell numbers were alsofollowed using a Coulter counter. These were used toestimate the degree of synchrony of the culture. Mostsynchronous cultures had a synchrony index of 0.35 to0.40 as calculated by the method described by Engel-berg (7).

Samples were taken at intervals by filtering por-tions of the culture onto Whatman GF/C glass fiberfilters, washing the cells first with cold buffer ascontained in the culture medium, and then withdistilled water. The soluble amino acids were thenextracted exhaustively with 60% ethanol. By thismethod the total cellular pool of soluble amino acidsis extracted. Identical results were obtained as withthe method using methanol described above or withother standard extraction methods (boiling water,cold trichloroacetic acid). Determinations of a-aminonitrogen and arginine were performed directly on theextracts, or the ethanol was evaporated and theresidue containing the amino acids was resuspendedin a small volume of distilled water. Protein was

determined in cells after their extraction by 5%trichloroacetic acid.

Pulse labeling. The radiochemicals were all ob-tained from the Radiochemical Centre, Amersham,England, and were used at the following specificactivities and concentrations: L-[U-_14C]arginine (2 to5 A.Ci/mmol, 5 x 10-' M), L-[U-14C]leucine (20ACi/mmol, 0.5 x 10- 3M), D-[U-14C]glucose (40 ICilmmol, 25 x 10-3M).

Radioactivity was determined in the 60% ethanol-soluble and insoluble cellular material by liquidscintillation spectrometry as described (35). Cellscollected and washed three times with 5 ml of coldmedium buffer on a Whatman GF/C glass fiber filterwere immersed and suspended together with the filterin the scintillator solution.

Determination of amino acids, protein, and totalnitrogen. Total amino acids (a-amino nitrogen) andarginine were determined as before (35). The purifica-tion of individual amino acids for quantitation anddetermination of specific radioactivities was carriedout by use of a Technicon TSM amino acid analyzer.The protein of the cells extracted either with 5%trichloroacetic acid or methanol was solubilized byincubation of the cells for 10 min at 95 C in 0.72 Nsodium hydroxide. Protein was then determined bythe method of Lowry et al. (16) using bovine serumalbumin as a standard. Total nitrogen was deter-mined in cells collected on Whatman GF/C glass fiberfilters and washed three times with 5 ml of coldmedium buffer, using the micro Kjeldahl method.

RESULTSChanges in cellular density during the

budding cycle. A cell population of an exponen-tially growing asynchronous culture of C. utiliswas centrifuged until the cells reached equilib-rium distribution (Fig. 1). The double cells withthe mother and a fully expanded daughter cell(Fig. 2) are recovered from the top of thegradients in the region of lowest density,whereas those cells with a small initial bud (Fig.3) are recovered from the bottom of the gradi-ents in the region of highest density. Cells fromother stages of the cycle equilibrate at interme-diate densities of dextrin. This fractionation ofa cell population of C. utilis is essentially thesame as that reported first for S. cerevisiae (33).The procedure can be conveniently used torapidly obtain selection synchrony from anexponentially growing asynchronous culture(11, 24, 33, 34). The fractions containing cells atpredominantly the same stage in the buddingcycle can be analyzed directly, or either the topor bottom fractions in the gradients can be usedas an inoculum for a synchronous culture. Thismethod for obtaining selection synchrony isbased exclusively on changes in cellular densitythat occur during the budding cycle. The use ofdextrin or other high-molecular-weight com-

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NURSE AND WIEMKEN

stepped density gradient

%w/v ofdontria

100

so

%eofgo"l type.

ns each fraectious *

40

20

20

CoO

1 24.O 1 24-4 1 25.7 1 27.0

-111471........................

-A6 30 47 la

1 2 3 4 Sfractions

FIG. 1. Fractionation of an exponentially growing C. utilis cell population on a stepped density gradient ofdextran. The culture was grown on nitrate (10 mM) and glycerol (1%) as the nitrogen and carbon sources,respectively. The proportion of cells with no buds or small buds and of cells with medium-sized or large budswere counted in each fraction.

FIG. 2 and 3. C. utilis cells selected by densitygradient centrifugation from an exponentially growingculture. Late-budding cells from the top layer of agradient (Fig. 2) and initial budding cells from thebottom layer (Fig. 3). Phase-contrast x550.

pounds, which exert negligible osmotic pressurefor preparing the gradients, is essential to pre-vent loss of water and shrinkage of the cells,eliminating the differences in density of cells atdifferent stages of the budding cycle (34). It alsoprevents disturbance of the cells due to osmoticshock. This method using isopycnic centrifuga-tion is fundamentally different, therefore, fromthat using sedimentation velocity centrifuga-tion separating cells according to their size (20,23).Changes in the soluble amino acids during

the budding cycle. The soluble amino acids

were studied in C. utilis growing exponentiallyon arginine as the sole nitrogen source. Underthese conditions arginine is known to accumu-late in the vacuolar storage pool (35, 36) andaccounts for nearly half of the total solubleamino acids (Table 1). Such a culture wasfractionated on density gradients, and the frac-tions obtained were analyzed for their solubleamino acid and protein contents (Fig. 4a). Thelate-budding cells from the top of the gradientsare found to have the highest soluble amino acidcontent per protein. This content decreases withincreasing density of the cells. A large propor-tion of this decrease can be accounted for bychanges in the soluble arginine.The same experiment was performed with

cultures grown on glycine or nitrate as solenitrogen sources, which have comparablegrowth rates (Table 1). As in the arginine-grownculture, in both the glycine- and nitrate-growncultures the soluble amino acid content perprotein was found to decrease with increasingdensity of the cells (Fig. 4b and c). A largeproportion of the decrease in the glycine-growncells is due to glycine, whereas in the nitrate-grown cells a number of different amino acidscontributes to the decrease observed.A disadvantage of this type of analysis is the

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AMINO ACIDS DURING C. UTILIS CELL CYCLE

TABLE 1. Soluble amino acids and generation time ofC. utilisa

Total Total solubleNitrogen Generation soluble amino acidssourceb time amino acids (%) as:"(10 mM) (min) (mmol/g of

protein) Arg Gly

Arg 120 2.43 48.7 0.4Gly 148 2.19 4.9 50.0NO 99 2.00 18.7 0.8

a Cells were growing exponentially on glucose (1%)at 26 C.

Arg, arginine; Gly, glycine.a b c

I

E IE

,

totalamino acids

2 ly k

123456 12345.. 67.t 2 3 4 5 6 1 2 3 4 5 6 7 2 3

. .

X 2 3 4 5 6

f rect ions

FIG. 4. Soluble amino acids of a C. utilis cellpopulation fractionated on stepped density gradientsof dextrin as in Fig. 1. The cultures were grown onarginine (a), glycine (b), and nitrate (c) as solenitrogen sources (10 mM each), with glycerol (1%) asthe carbon source. Individual fractions were washed,fixed in methanol, and extracted with water asdescribed in Materials and Methods. The solubleamino acid content per protein was determined foreach fraction. Prevailing cell types in the fractions aregiven at the top of the graphs.

delay of up to 1 h between harvesting of theculture and extraction of the cells collected fromthe density gradients. Although the cells werekept at 0 to 4 C throughout the fractionationprocedure, this delay could result in changes ofthe soluble pools by reduced metabolism stilltaking place at these low temperatures. Tocheck this possibility, a synchronously growingculture was analyzed; from such a culture, cellsat different stages of the cell cycle could beextracted immediately after sampling.An arginine-grown culture was centrifuged in

a density gradient, and the fraction containingpredominantly initial budding cells (Fig. 3) wasused as an inoculum for a synchronous culture.During this culture samples were taken for the

estimation of the soluble total amino acids,arginine, glutamate, and ornithine. These areplotted per milliliter of culture in Fig. 5. All fourincreased in a step during the phase of budextension and remained constant from cell divi-sion until after the next phase of bud initiation.These results confirmed that the soluble aminoacids were greatest in amount in cells of leastdensity with the largest buds and were smallestin cells of greatest density with only tiny buds.Another arginine-grown culture was centrifugedas above, and all the fractions were mixedtogether and used as an inoculum for an asyn-chronous culture. In this culture the total solu-ble amino acids increased exponentially (datanot shown), demonstrating that the fluctuationsobserved in synchronous cultures were not anartifact of the selection procedure.Changes in arginine uptake and catabo-

lism during the budding cycle. In the course ofthe budding cycle the soluble amino acids maychange as a result of changing rates of aminoacid pool formation or utilization, or both.These possibilities were examined using iso-topic labeling techniques. Portions of a culturegrowing synchronously on arginine as the sole

0 -O00X | -

10

200-7E

5 %..

10

10

3

7UE5~~~~~~~

C)

100 200 minFIG. 5. The soluble total amino acids (U), arginine

(0), glutamate (A), and omithine x10 (V) in asynchronous culture of C. utilis grown on arginine (5mM) as sole nitrogen source, and glucose (1%) ascarbon source. The cell numbers were countedthroughout the culture (0). Cell types are given at thetop of the figure.

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nitrogen source were pulse labeled for 10 minwith arginine. The rate of uptake of label intothe total cellular material and the 60% ethanol-insoluble material was determined (Fig. 6). Thetotal protein content was also followed through-out the culture. It can be seen from Fig. 6 thatthe rate of uptake of arginine doubles in a singlestep during the period of bud expansion. Thisstep contrasts with the exponentially increasingrate by which label from arginine was incorpo-rated into the insoluble material. The totalprotein content of the culture also increasesexponentially. The rates of labeling after a10-min L- [U- 14C ]arginine pulse were followedfor two of the soluble aihino acids, arginine andits first catabolic product ornithine. Shorter andlonger pulses had already established that therewas little lag before the entry of label into bothamino acids and that, with a 10-min pulse,neither amino acid became saturated. In thecourse of the synchronous culture the rates offormation of both amino acids increased almostsimultaneously in a single step (Fig. 7). Theuptake and catabolism of arginine appears,therefore, to be closely linked. However, thedegree of synchrony may not have been highenough to distinguish these two events if theyclosely followed each other in the cell cycle.This would also explain why no preferentialaccumulation of arginine could be detected

7

6

5

4h.3

3 3u

iEE 2

u

0 100 200 minFIG. 6. Synchronous culture of C. utilis growing on

arginine (5 mM) as sole nitrogen source and glucose(1%) as carbon source. Portions of the culture werepulse labeled at intervals for 10 min with L-[U-'4C]ar-ginine. The total uptake of label into the cells (0), thelabel incorporated into 60% ethanol-insoluble mate-rial (A), and the protein (U) were estimated. Celltypes are given at the top of the figure.

200

ioo -

U

E.

(.1 50h

30F

201 a a I100 200 min

FIG. 7. Synchronous culture of C. utilis grown andpulse labeled as in Fig. 6. The label was measured insoluble arginine (0) and omithine x5 (A). Cell typesare given at the top of the figure.

during the synchronous culture illustrated inFig. 5, as was observed in the lightest cellsanalyzed directly from the gradient (Fig. 4).As arginine was the sole nitrogen source, a

stepped increase in the rate of uptake of argi-nine should result in a linear increase in totalcellular nitrogen, doubling in rate once everycell cycle. This was checked by following thetotal cellular nitrogen in a synchronous culture(Fig. 8). The nitrogen content of the cellsincreased continuously, and has been inter-preted in Fig. 8 as showing a linear increasedoubling in rate during bud expansion. It isdifficult to distinguish between linear and expo-nential patterns of increase, but clearly the dataare not inconsistent with a stepped increase inthe rate of uptake of the exogenous nitrogensource.Changes in glucose uptake and amino acid

anabolism during the budding cycle. Thesimultaneous increases in both the rate ofcatabolism of arginine and in the amounts ofother soluble amino acids during bud expansionsuggest that this is a period of increased aminoacid biosynthesis. Since 90% of the solublecarbon is found in the amino acids (6), in-creased amino acid biosynthesis should result inan increase in the label recovered in solublematerial extracted from cells pulse labeled with[U-'4C]glucose. This was checked by pulse la-beling portions of a synchronous culture for 10min with [U-"C ]glucose. The rate of uptake oflabel into the total cellular and the 60% ethanol-

- - oOD cc0 o -.00

CP-o0 - -00-*0C

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AMINO ACIDS DURING C. UTILIS CELL CYCLE

soluble material was determined (Fig. 9). Aspredicted, the uptake of label into the 60%ethanol-soluble material increases as a stepduring bud expansion. The rates of labelingafter a 10-min pulse with [UL-4C]glucose werefollowed in more detail for five soluble aminoacids-glutamate, glutamine, alanine, serine,and threonine. Shorter and longer pulses hadalready established that there was little lag

0

EC00,0b-

C0,a

801-

60 F

40 F

20r

v

100 200 minFIG. 8. Increase of total nitrogen in a synchronous

culture of C. utilis growing on arginine (5 mM) as solenitrogen source and glucose (1%) as carbon source.Cell types are given at the top of the figure.

0L)

E

E

100 200 minFIG. 9. Synchronous culture of C. utilis growing on

arginine (5 mM) and glucose (0.5%). Portions of theculture were pulse labeled at intervals with[14C]glucose for 10 min. The label in the total cellular(-) and in the 60%o ethanol-soluble material (0) weredetermined. Cell types are given at the top of thefigure.

a)

E

before the entry of label into all the aminoacids, and that with a 10-min pulse no aminoacid became saturated. In the course of thesynchronous culture, the labeling rates of all ofthem increase almost simultaneously in a singlestep during bud expansion (Fig. 10). The uptakeand catabolism of arginine appear, therefore, tobe closely linked to a general increase in aminoacid biosynthesis. Despite the increased de-mand of carbon skeletons for amino acid biosyn-thesis during bud expansion, the uptake ofglucose into the total cellular material increasesexponentially throughout the entire buddingcycle (Fig. 9).Protein degradation as a source of soluble

amino acids during the budding cycle. In caseprotein degradation was contributing to theincrease in amount of soluble amino acidsduring the budding cycle, the contribution ofprotein degradation to amino acid pool forma-tion was measured in a synchronous culture inwhich the proteins had been prelabeled with

00 00 -OCD - o00

100 200 minFIG. 10. Synchronous culture of C. utilis grown

and labeled as in Fig. 9. The label was measured inglutamate (U), threonine xlO (x), alanine (A),glutamine (0), and serine (V). Cell types are given atthe top of the figure.

In-o o- CD _

-* O0 - 0C -. o' - *00

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NURSE AND WIEMKEN

L- [U-14C]leucine. To obtain low labeling of theleucine pool in the cells, and good labeling of theproteins, an asynchronous culture was labeledfor 30 min with L-[U-_4C]leucine and was thenwashed free of label and chased for 60 min withL- [I4C Ileucine. This culture was then harvestedand fractionated to obtain the synchronousinoculum. The label in insoluble material andthe total protein were followed during the syn-chronous culture. As can be seen from Fig. 11,there was no loss of label from the insolublematerial during the budding cycle. It should bestressed that this technique could not havedetected either very low protein turnover, ortumover which involved an immediate reincor-poration into the protein of the released aminoacids. However, in both these cases, proteindegradation would contribute little to the for-mation of the amino acid pools.

DISCUSSIONThe results presented in this paper demon-

strate that the amount of soluble amino acids insynchronous cultures of C. utilis increases ab-ruptly during the phase of bud expansion (Fig.5). This phase occupies about one-half of thecell cycle in rapidly growing cultures, and lastsfrom bud initiation until the formation of largedouble cells. During the other half of the cyclethe amount of soluble amino acids remainsvirtually constant in synchronous cultures. Thisobservation, suggesting a fluctuation of thetotal pool of amino acids in the cells once every

10

0

'EE50.

0 100 200FIG. 11. Synchronous culture of C. utilis grown on

5 mM arginine. Cells were prelabeled with L-[U-14C]leucine for 30 min, and were chased with L-[12C]leucine for 60 min before an inoculum wasseparated for the synchronous culture on densitygradients. The label incorporated into the 60%ethanol-insoluble material (0) and the protein con-tent (U) were followed during the synchronous cul-ture. Cell types are given at the top of the figure.

cell cycle, is supported by measurements of thecold trichloroacetic acid-soluble pools in singlecells of Schizosaccharomyces pombe made byinterference microscopy by Mitchison andCummins (19). The observation is contradictedby the reports of Carter et al. (2) and Stebbing(26, 27). These authors suggested that thesoluble amino acids increase exponentially insynchronous cultures of S. cerevisiae and S.pombe. However, in our opinion their data arenot inconsistent with a step increase in thesoluble amino acids also for their yeasts. Thestandard error calculated by Stebbing (27) forhis data is high, and from the data given in thepaper of Carter et al. (2) it is difficult toestablish the precise pattern of increase of thesoluble amino acids. Furthermore, their cul-tures may not have been in steady-state growthconditions; this is indicated by the gradualincrease of the soluble arginine per dry weight ofthe cells during the synchronous culture of S.cerevisiae (2), and the quite dramatic increaseof the soluble alanine at the end of the synchro-nous culture of S. pombe (27). The pools mayhave been altered by the osmotic pressureexerted by the sucrose gradients used to preparethe synchronous cultures by the technique ofsedimentation velocity centrifugation.A large number of studies on synchronous

cultures of yeast are concerned with regulatorymechanisms of enzyme synthesis during the celldivision cycle, and two enzymes specificallyinvolved in arginine catabolism have been in-vestigated (2). However, because it is now wellestablished in yeast (35, 36) and in Neurospora(28, 31) that the soluble amino acids are exten-sively compartmentalized at the subcellularlevel, we consider it impossible from measure-ments of the total cellular amino acid pools tocontribute anything to the debate on the oscilla-tory repression and sequential transcriptionmodels for the control of enzyme synthesisduring the cell cycle (10, 18).The increase in amount of the soluble amino

acids is simultaneous with the expansion of thevacuoles and a drop in cellular density (34).This strongly suggests that the large vacuolarstorage pools (35, 36) are laid down at this time.As protein and ribonucleic acid synthesis in-crease continuously, these storage pools may bereleased back into the cytoplasm later in thecell cycle if the rate of utilization of solubleamino acids exceeds their rate of formation. Inthis context it is interesting that the vacuolesshrink and divide during bud initiation (34). Ifthe vacuolar storage pool is utilized during onephase of the cycle, a drop would be expected tooccur in the total of soluble amino acids insynchronous cultures. This can not be clearly

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recognized in the experiments reported here(Fig. 5), probably because the degree of syn-

chrony is too low. In synchronous cultures of S.cerevisiae, however, the amino acid pool was

actually found to decrease during bud initiation(A. Wiemken, Ph.D. thesis Nr 4340, Swiss Fed-eral Institute of Technology, Zurich, 1969).The step increase in the amount of the soluble

amino acids is accompanied by a sudden dou-bling in the rate of uptake of the sole nitrogensource, arginine (Fig. 6). The doubling in rate ofuptake is consistent with the linear increase intotal cellular nitrogen (doubling in rate once

every cell cycle) that was also observed (Fig. 8).Although, from our data, it is difficult todistinguish unambiguously between exponen-

tial and linear patterns of increase, it is quiteclear that there are no step increases in totalcellular nitrogen, such as have been reported forsynchronized cultures of S. cerevisiae (29, 37).This step pattern of increase may have beencaused by the feeding and starvation cycleswhich were used to synchronize the cells inthese studies. Meyenburg (17) has also observeda discontinuous increase in total cellular nitro-gen during synchronous cultures of S. cerevisiaegrown in a chemostat under carbon-limitingconditions.

Carter and Halvorson (3) have recently re-

ported that transport activities for variousamino acids increase as steps during synchro-nous cultures of S. cerevisiae. These studies,however, differ from ours, since the cells were

grown on ammonia as the sole nitrogen source,and the transport activities of the cells weremeasured during transient conditions after ad-dition of the amino acid concerned.Concomitant with the increase in amount of

the soluble amino acids and the step increase ofthe arginine uptake rate is a sudden doubling inthe rates of both arginine catabolism (Fig. 7)and other amino acid biosynthesis (Fig. 10).Despite this increase in the rate of formation ofthe soluble amino acids, there is no abruptincrease in the rate of their utilization. Proteinsynthesis and, to a minor extent, ribonucleicacid synthesis are the greatest drains on thesoluble amino acids, and both were found toincrease exponentially throughout the cell cycle(Fig. 7) (26, 34, 37, 38). The sudden doubling inthe rate of amino acid biosynthesis would resultin an increased demand for carbon skeletons,and since the uptake rate of the carbon andenergy source glucose increases exponentiallyduring a synchronous culture (Fig. 9), theremay be a diversion of carbon away from energy-producing reactions during bud initiation, intobiosynthetic reactions during bud expansion.This situation could result in a periodic fluctua-

tion of respiratory activity during synchronouscultures, and indeed this has been observed inS. cerevisiae (15, 17, 34, 37).

If metabolic reactions are substrate limited invivo (1, 25), then the rate of amino acid metabo-lism may be dependent on only a few substrate-saturated pacemaker reactions (13). In thiscase, the rate of amino acid metabolism couldbe instantly increased if the rates of these fewpacemaker reactions were simultaneously dou-bled. An effector molecule involved in nitrogencatabolite repression (21, 32) could be responsi-ble for the coordination of the doubling in ratesof pacemaker steps in amino acid metabolism.One possible candidate for a pacemaker is theuptake mechanism of the nitrogen source. Theobserved doubling in the rate of arginine up-take may be the consequence of increase syn-thesis of an arginine permease (9), an increasein the space available for permease insertioninto the plasmalemma (12), or a relief of trans-inhibition (4, 14).

It is amazing how quickly the doubling of therate of amino acid metabolism follows the doub-ling of the rate of amino acid uptake in thecell cycle. Small effector and metabolic poolslocated in the cytoplasm (35) could enable thecell to respond instantly to changes triggered bythe environment or by the internal cell cycleprogram. However, the depletion of these poolscould be prevented by the buffering action ofthe large vacuolar storage pools (35). Thus, thespatial and temporal compartmentation of thesoluble amino acids within the yeast cell mayplay an important role to insure that amino acidmetabolism is precisely and rapidly regulatedwhile remaining substantially independent ofthe cellular environment.

ACKNOWLEDGMENTSWe would like to express our sincere thanks to S. Howitt

for his invaluable assistance with the amino acid analyzer andto A. P. Sims and B. F. Folkes for their help throughout thiswork. We are grateful to P. Matile for reading the manuscript.

P.N. was the recipient of a Science Research Council(SRC) studentship. The work was also supported by an SRCresearch grant.

LITERATURE CITED1. Atkinson, D. E. 1969. Limitation of metabolite concentra-

tion and the conservation of solvent capacity in theliving cell, p. 29-43. In B. L. Horecker and E. R.Stadtman (ed.), Current topics in cellular regulation,vol. 1. Academic Press, Inc., New York.

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