© 1999 Macmillan Magazines Ltd

trols rightward looping of the heart and is notaffected by Pitx2 (see the reports by Lu et al.10

and Lin et al.11 on pages 276 and 279 of thisissue). In the end, then, the exact timing ofexpression, rate of diffusion and relativethresholds of activity of all these ligands andantagonists must combine to determinepatterns of cell fate.

Investigating these interactions is quiteenough to be going on with, but researcherswill also be keen to isolate Caronte in themouse. Surprising differences have alreadycome to light between the mouse and thechick (Fig. 1), including the roles of Shh12,13

and cilia on node cells14. In the mouse,FGF-8 is needed for left-sided expression ofNodal13, whereas it is involved on the right inthe chick. Conversely, Esteban et al.1 describea second transcription factor, Nxk3.2, whichis induced by Nodal on the left in the chick,but appears on the right in the mouse15. Is it a

paradox that the final asymmetries in bodyplans are very well conserved across species,yet their developmental pathways are not? ■

Tim King and Nigel A. Brown are in theDepartment of Anatomy and DevelopmentalBiology, St George’s Hospital Medical School,University of London, Cranmer Terrace, LondonSW17 0RE, UK.e-mail: nbrown@sghms.ac.uk1. Esteban, C. R. et al. Nature 401, 243–251 (1999).

2. Harvey, R. P. Cell 94, 273–276 (1998).

3. Zhu, L. et al. Curr. Biol. 9, 931–938 (1999).

4. Yokouchi, Y. et al. Cell 98, 573–583 (1999).

5. Piccolo, S. et al. Nature 397, 707–710 (1998).

6. Meno, C. et al. Genes Cells 2, 513–524 (1997).

7. Thisse, C. & Thisse, B. Development 126, 229–240 (1999).

8. Bisgrove, B. W. et al. Development 126, 3253–3262 (1999).

9. McMahon, A. P. & Perrimon, N. Cell 97, 13–16 (1999).

10.Lu, M.-F. et al. Nature 401, 276–278 (1999).

11.Lin, C. R. et al. Nature 401, 279–282 (1999).

12. Izraeli, S. et al. Nature 399, 691–694 (1999).

13.Meyers, E. N. & Martin, G. R. Science 285, 403–406 (1999).

14.Cooke, J. BioEssays 21, 537–541 (1999).

15.Schneider, A. et al. Curr. Biol. 9, 911–914 (1999).

on clouds and climate has been discussed atlength during the past few years as a possiblemasking effect on greenhouse warming6. Butbecause of the complexity of the processesinvolved and the difficulty of validating theo-retical estimates by measurements, the mag-nitude of this indirect aerosol effect on climatehas remained uncertain. Indeed, an assess-ment of climate change6 includes the corre-sponding estimate of global climate forcing asa wide range (0 to 11.5 W m12), with no cen-tral value given. This compares with the esti-mate of positive forcing of about 2.4 W m12

due to the increase in greenhouse gases.What Facchini et al.1 have done is to high-

light another factor in the Köhler equation,surface tension, and apply an empiricalapproach to estimate its magnitude. Theirargument is that the presence of surfactantorganic material could lower the surfacetension of the wetted aerosol particles so thatthe critical supersaturation Sc decreases andallows more particles to become activatedand grow to cloud droplets. This, again,would make the clouds brighter and lead to anegative climate forcing.

Although they are not the first to pointout this possibility (Li et al.7 have done so, forinstance), Facchini and colleagues’ estimateof the magnitude of this effect represents anotable contribution in that it is based on anactual measurement of surface tension in asample of water collected from ambientclouds. In this way the pollutants in the watersample, notably the organic compounds, arethose present in the real atmosphere ratherthan the selected compounds studied in pre-vious investigations7,8. On the other hand,unlike Li et al.7, Facchini and colleagues donot consider other influences of organicpollutants on Sc. For example Sc could in-crease due to a reduction in the molality ofthe solution caused by the high molecularweight of the organics.

Facchini et al. collected cloud water fromfog (that is, clouds at ground level) in the PoValley in Italy. They then partly evaporatedthe sample to simulate the larger concentra-tions of the organic compounds occurring inthe droplets at their activation threshold.Assuming equilibrium conditions apply,which may not be perfectly true9, they intro-duced the modified value of the surface ten-sion into Köhler’s equation and derived anestimate of the resulting decrease in Sc. Usinga ‘back-of-the-envelope’ calculation, theywent on to estimate an approximate magni-tude of the global climate forcing caused bythe change in surface tension due to organicsurfactants, assuming that these are all man-made.

The resulting figure (almost 11 W m12)must be looked on with caution, however. Forone thing, as the authors acknowledge, it islikely to be an overestimate because theyassume all stratus clouds to be equally affect-ed by those man-made organic pollutants

news and views

NATURE | VOL 401 | 16 SEPTEMBER 1999 | www.nature.com 223

Soapy components of air pollutantslower the surface tension of tiny, wet-ted, aerosol particles. This could make

the particles more likely to turn into clouddrops, through condensation of watervapour on their surface, and lead to a largernumber of cloud drops of smaller size. Theupshot — the clouds become more efficientreflectors of sunlight and thereby tend tocool the Earth’s surface. This chain of eventshas been investigated by Facchini et al. (page257 of this issue1), who have applied a novelempirical approach to one of the links.

Since the late nineteenth century, it hasbeen known that clouds consist of smallwater droplets (or ice crystals) and that thecondensation of water vapour to form suchdroplets takes place on pre-existing aerosolparticles called cloud condensation nuclei(CCN). The conventional wisdom for manyyears has been that there are always enoughCCN in the air — from soil dust, sea sprayand other natural sources — to allow cloudsto form whenever the cooling of ascendingair parcels by adiabatic expansion createssupersaturation. Additional man-madesources were not believed to have any effect.Today we know that this picture is too sim-ple. Both the physical characteristics2,3 andamount of cloud4 can be affected by man-made changes in the CCN population.

In 1921 the Swedish meteorologist Hild-ing Köhler formulated the now classic theoryof how CCN can be ‘activated’ and grow bycondensation to cloud droplets5. Accordingto Köhler’s equation, supersaturation hasto exceed a critical value, Sc, not much above

100% saturation, before a droplet starts togrow spontaneously to cloud-droplet size.It turns out that Sc is a function of, amongother things, the concentration of solute inthe wetted aerosol particle and its surfacetension. By adding soluble material to theatmosphere (for example sulphate particlesderived from industrial emissions of SO2) weare now increasing the number and the effi-ciency of CCN, and thereby increasing theconcentration of droplets in the clouds ofpolluted regions2. As a result, the cloudsbecome brighter — because of an increasedsurface area per volume of water — and mayalso be longer lived.

This particular route of human influence

Atmospheric chemistry

Clouds and climateHenning Rodhe

Figure 1 Enigmatic clouds — stratus, seen overMonterey Bay, California. In the work discussedhere, Facchini et al.1 estimate the extent to whichorganic pollutants can increase surface tensionin cloud droplets, leading to an increase indroplet number and thereby to a higher level ofscattering of incoming solar radiation. Thesignificance of the effect on a global scale will,however, need further evaluation — clouds arecomplicated subjects of study.

CO

RB

IS/J

ON

AT

HA

N B

LAIR

© 1999 Macmillan Magazines Ltd

present in the Po Valley, even clouds occur-ring in remote marine areas. Marine stratusclouds have turned out to be the most suscep-tible to this kind of impact. Furthermore, ifthey had included other effects of the organicpollutants, including a possible increase in Sc

due to the organic material being dissolved inthe droplets, the resulting climate forcing bythe organic aerosol might even have turnedout to be positive7. So at this stage it would bepremature to take the authors’ estimate of cli-mate forcing at face value.

Nevertheless, their calculation doesdemonstrate the potential importance of thesurfactant effect on cloud brightness and cli-mate. This is another step towards a betterunderstanding of the complex mechanismsaffecting the climate system. Obviously,much more has to be learnt about thesources, occurrence and effect on clouds ofthese kinds of organic compounds. But

because of the complexity of the microphysi-cal and chemical processes involved in cloudformation, we may never be able to describethem in full detail. So such microphysicalstudies also have to be complemented bylarger-scale (‘epidemiological’) measure-ments of cloud brightness and cloud dropletradii from aircraft and satellites. ■

Henning Rodhe is in the Department ofMeteorology, Arrhenius Laboratory, University ofStockholm, S-106 91 Stockholm, Sweden.e-mail: rodhe@misu.su.se1. Facchini, M. C., Mircea, M., Fuzzi, S. & Charlson, R. J. Nature

401, 257–259 (1999).

2. Twomey, S. J. Atmos. Sci. 34, 1149–1152 (1977).

3. Boers, R., Ayers, G. P. & Gras, J. L. Tellus B 46, 123–131 (1994).

4. Albrecht, B. A. Science 245, 1227–1230 (1989).

5. Köhler, H. Meteorol. Z. 38, 168–171 (1921).

6. Houghton, J. T. et al. (eds) Climate Change 1995: The Science of

Climate Change (Cambridge Univ. Press, 1996).

7. Li, Z. et al. J. Atmos. Sci. 55, 1859–1866 (1998).

8. Hansson, H.-C. et al. J. Atmos. Chem. 31, 321–346 (1998).

9. Chuang, P. Y. et al. Nature 390, 594–596 (1997).

stalls (B. Maier, École Normale Supérieure;C. Bustamante, Univ. California, Berkeley).

This behaviour is consistent with thestretching that causes a reduction in theentropy of the template, which in turn facili-tates DNAp translocation. At higher ten-sions, however, the enzyme has to workincreasingly hard to pull the template strandtogether and fit it into the ‘closed’ conforma-tion that allows nucleotide incorporation;replication therefore proceeds at a slowerrate before stalling. Intriguingly, a furtherincrease in applied tension gets the DNApgoing again — but in the opposite direction.That is, the enzyme reversibly switches fromreplication to exonuclease digestion, an abil-ity that forms the heart of its in-built ‘proof-reading’ mechanism (Bustamante).

The action of DNAp is an essential step inpassing genetic information to daughtercells. RNA polymerase (RNAp), by contrast,initiates gene expression. As with DNAp, theactivity of individual RNAp molecules ishighly variable and can be stalled reversiblyby applying a force of over 30 pN or so16. But amore pressing question concerning RNApactivity is how so-called ‘terminator’sequences influence gene expression. Termi-nators stop RNAp transcription at predeter-mined positions on the DNA with efficien-cies ranging from nearly 0% to 100%. Single-molecule experiments (Fig. 1c) show thatonce an enzyme pauses, it has reached itspoint of no return — pausing is necessary aswell as sufficient to cause termination(J. Gelles, Brandeis Univ.). This leaves thequestions of how accessory factors influencecompetition between nucleotide additionand entry into a paused state, and whether itmight be possible to control this competi-tion for medical purposes.

DNA topoisomerase II (topo II), which isresponsible for untangling replicated chro-mosomes before they are pulled apart duringcell division, has also attracted the attentionof physicists and pharmaceutical companies.The enzyme cuts a DNA segment, moves theuncut segment through the gap in the strandand then re-ligates the cleaved DNA, releas-ing two supercoils. This activity, driven byATP hydrolysis, has now been observedexperimentally (Fig. 1d) by following thediscrete ‘jumps’ associated with the release ofindividual helical twists from a supercoiledDNA strand stretched between a glass sur-face and a magnetic bead (T. Strick, ÉcoleNormale Supérieure). The observation oftopo II clamping onto thermally activatedDNA cross-overs in the absence of ATPshows both that the enzymatic cycle can bedissected into its constituent parts, and thatthis first step is not chemically fuelled.

When applied to the right questions, sin-gle-molecule biophysics provides answersthat are difficult or impossible to obtain frombulk experiments. It is now possible to lookbeyond the ensemble average, and to dissect

news and views

NATURE | VOL 401 | 16 SEPTEMBER 1999 | www.nature.com 225

Biophysics

Singular take on molecules Magdalena Helmer

What is life? At its most basic, it is thesubtle interplay of large numbersof proteins that interact through

enzymatic reactions and complex signallingpathways. The description of this molecularmachinery becomes more detailed by theday, yet our understanding of biologicalprocesses at the most fundamental, mecha-nistic level remains poor. High-resolutionstructural images are of great help, of course,but what are needed are functional assaysthat can interrogate the inner workings ofcomplex biological machines. Such assays doexist, and as they become more sophisticatedwe can expect a transformation in ourknowledge of single-molecule behaviourin biological systems — that, at least, wasthe message to emerge from a gathering of‘single-molecule biophysicists’ at the firstconference* dedicated to their cause.

Technical advances have made it possibleto probe the strength of chemical bonds1,2,the elastic and mechanical properties of mol-ecules3–5, and the conformational changesand forces that drive molecular motors at thesingle-molecule level6–9. Advanced opticalprobes enable the direct tracking of cellularprocesses10 and ion-channel activity (E.Isacoff, Univ. California, Berkeley; P. R.Selvin, Univ. Illinois), and ever-more inven-tive spectroscopic methods are being usedto unravel the kinetics of enzymatic11,12 andphotobiological reactions13, and the inter-mediates and energy-transfer processesinvolved. The ability to discern functional

(and presumably structural) heterogeneityin the properties of biomolecules andobserve individual interactions in unprece-dented detail will, it is hoped, ultimately leadto a mechanism-based and more quantita-tive understanding of molecular biology.

Discussions at the meeting took in allof these topics14, and more — research withDNA, for instance, which physicists use tostudy the dynamic behaviour and elasticityof polymers15 because it is fairly large, con-formationally stable, and relatively easy toimage and handle. Of late, however, physi-cists’ interest has turned to topics such as theunwinding of duplex DNA by the helicaseenzyme rep, the key step preceding read-outof the genetic blueprint. Using fluorescenceresonance energy transfer, the activity of asingle rep helicase can be monitored (T. Ha,Stanford Univ.). As the distance betweendonor and acceptor fluorophores on the twostrands of a duplex increases, the efficiencyof the energy transfer decreases and soreveals the extent and time-course ofunwinding.

Once short segments of single-strandedDNA are generated inside a dividing cell,they can be replicated by DNA polymerase(DNAp), a molecular motor that catalysesDNA synthesis and uses part of the excessfree energy released to translocate along thetemplate strand. This activity is susceptibleto mechanical force: when increasing ten-sion is applied to the template strand (Fig.1a,b, overleaf), DNAp replication speeds upuntil the tension reaches a value of about 5 to6 pN; it then slows down rapidly and finally*Single Molecule Biophysics, Tours, France, 8–15 July 1999.