Giant Marine Cyanobacteria ProduceExciting Potential PharmaceuticalsCyanobacteria produce an array of exceptionally potent toxins,some with promising anticancer or other anti-disease activities

William H. Gerwick, R. Cameron Coates, Niclas Engene, Lena Gerwick,

Rashel V. Grindberg, Adam C. Jones, and Carla M. Sorrels

Our standard procedure for collect-ing specimens of tropical marinecyanobacteria as potential sourcesfor novel pharmaceuticals requiressome of us to dive about 15 meters

below the surface and 1 meter above the tropicalsea floor, scanning the underwater landscape forgiant colonies of marine cyanobacteria. In suchsettings, benthic cyanobacteria grow as nearlypure strains—forming tufts, mucilages, andmats that adhere to coral, algae, and marineinvertebrate substrates (Fig. 1).

Because cyanobacterial colonies grow in suchprofusion, we can collect them by hand in largeenough quantities to investigate their chemical,pharmacological, and genetic properties. Thisapproach contrasts with how microbiologists

typically come to know particular microorgan-isms, often depending on elaborate manipula-tions and culture procedures to acquire ade-quate materials to study. In our case, however,the substantial capacities of some cyanobacteriafor photosynthesis and nitrogen fixation enablethem to reach localized population sizes that areinconceivable for so many other species of themicrobial world. Of course, this physical staturealso makes them subject to predation by fish,mollusks, and a variety of mesograzers. To de-fend themselves in this predator-rich environ-ment, cyanobacteria produce an array of excep-tionally potent toxins—often the very materialsthat are of such keen interest to us.

Building in part on the pioneering efforts ofthe late Richard Moore during the 1970–1990s

at the University of Hawaii, we are continu-ing to determine structures and informationabout the biosynthesis and activities of nat-ural products from marine cyanobacteria,working at Scripps Institution of Oceanog-raphy after moving from Oregon State Uni-versity. These insights are helping us tobetter appreciate the role that marine inver-tebrate-associated cyanobacteria play inproducing natural products in marine envi-ronments.

Giant marine cyanobacteria, producingsecondary metabolites with unusual struc-tures and potent activities against cancerand other diseases, thus are an exciting areafor microbial research. The genes and bio-chemical pathways responsible for makingthese metabolites are equally worthy ofstudy and will help to address a series ofpuzzling questions regarding their natural

Summary

• Giant cyanobacteria that grow in marine envi-ronments produce an array of natural products,some of them potent toxins with promisinganticancer or other anti-disease activities.

• Filamentous marine cyanobacteria may yieldpromising natural products and, of these, Sub-section III cyanobacteria are the source ofnearly half the 800 reported compounds.

• Such filamentous forms integrate features fromthe polyketide synthase and nonribosomal pep-tide synthetase pathways.

• Marine invertebrate species are remarkable forhow extensively they associate with marine mi-crobiota, which produce many of the naturalproducts that chemists isolate from those inver-tebrate sources.

William H. Gerwickis Professor ofOceanography andPharmaceutical Sci-ences, R. CameronCoates is SeniorResearch Assistant,Niclas Engene isdoctoral student,Lena G. Gerwick isResearch Scientist,Rashel V. Grindbergis doctoral candi-date, Adam C.Jones is doctoralcandidate, andCarla M. Sorrels isdoctoral candidateat the Center forMarine Biotechnol-ogy and Biomedi-cine, Scripps Insti-tution ofOceanography, andthe Skaggs Schoolof Pharmacy andPharmaceutical Sci-ences University ofCalifornia San Di-ego, La Jolla, Calif.

Volume 3, Number 6, 2008 / Microbe Y 277

chemistry, such as how microorganisms convertmethyl to trichloromethyl groups; how acety-lenic bromide groups are made; and what reac-tions form pendant carbon atoms at the C-1positions of polyketides, some of which appearas cyclopropyl rings and others as vinyl chloridearrangements.

Marine Settings a Key Source of

Potentially Therapeutic Natural Products

Marine bacteria have contributed at least 15natural products that are being evaluated inrecently concluded or in continuing clinical tri-als, especially as anticancer therapeutics. Interms of the sources of the bacteria from whichthose promising materials were collected, ma-rine invertebrates such as sponges, tunicates,and mollusks account for three-fourths of thesepromising molecules.

However, rigorously determining the realmetabolic sources of these metabolites some-

times requires further analysis. In sev-eral cases, seemingly invertebrate-derived molecules instead trace tobiosynthetic pathways that are associatedwith individual species of microorgan-isms or sets of microorganisms, includingheterotrophic bacteria, cyanobacteria,and fungi. Furthermore, when one pre-dicts the ultimate source of recently eval-uated clinical trial agents based on chem-ical class and likely biosynthetic pathwayfor making them, many such agents (15of 20) are products of single microbialgenetic capacities. Thus, the potential ofthe sea to yield useful secondary metabo-lites rests not so much with its diversemacro-life forms, but rather, with its in-credible microbiota.

For the nearly 800 compounds re-ported from marine cyanobacteria (Fig.2), the more advanced species such asthe filamentous forms tend to yieldpromising natural products. Of these,Subsection III cyanobacteria, accordingto bacteriological systems of phylog-eny, or the order Oscillatoriales by phy-cological delineation, are the source ofnearly half the reported compounds.From the 17 genera in Subsection III, or9 genera currently listed in the Oscilla-toriales, Lyngbya dominates, withnearly 300 distinct substances coming

from this single genus. There are 12 marinespecies in this genus, and 236 compounds areascribed to L. majuscula and a further 11 to L.bouillonii.

In contrast, other species of Lyngbya contrib-uted only a few metabolites; however, speciesidentification is rudimentary, making it likelythat many sources are not identified to the spe-cies level because environmental factors affectmorphological features such as growth form,coloration, and cell size measurements, render-ing them taxonomically unreliable.

We recently began identifying cyanobacte-ria based on 16S rDNA internal transcribedspacer (ITS) sequences and other housekeep-ing genes. This approach is revealing muchgreater genetic diversity within this groupthan is generally appreciated. Many new spe-cies and perhaps genera await taxonomic dis-covery—especially for isolates from marineenvironments because the preponderance of

F I G U R E 1

(A) Collection of filaments of a marine cyanobacterium into plastic collection bags usingSCUBA off the coast of Papua New Guinea. (B) Appearing as electric orange gelatinousorbs, colonies of the marine cyanobacterium Schizothrix sp. from reefs near New Britain,Papua New Guinea. (C) Cultured strain of Lyngbya majuscula 3L in 10 L of BG11 medium,and (D) cultured L. majuscula 3L at high magnification showing that the filaments arecomposed of coin-shaped cells (6.3 �m long by 60 �m diameter) and surrounded by apolysaccharide sheath.

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Gerwick: Water Is Loved for Leisure and as a Source for Natural ProductsWilliam Gerwick was about eightyears old when he first saw aniridescent thread of seaweedshimmering below the surface in atidal pool in Pacifica, a coastaltown south of San Francisco,Calif. He was with his father onone of their outings that kindledhis early interest in the ocean.“We would go to some of the tidepools outside San Francisco andpoke around, and then havebreakfast at the local fishermens’coffee shop,” he recalls.

The memory of that sliver ofshining seaweed—and a desire tounderstand why it glowed underwater, but not outside it—eventu-ally led to his first scientific report,“Structural, chemical and ecolog-ical studies on iridescence in Iri-daea (Rhodophyta),” publishedin the Journal of Phycology in1977. It was based upon an un-dergraduate research project thathe conducted at the University ofCalifornia, Davis (UCD), withNorma Lang, now professoremerita. “This particular type ofseaweed has a multilayered cuticleon its outer surface that workslike a very sophisticated soapbubble,” Gerwick says. “It re-flects certain wavelengths of lightwith brilliant intensity.” Born andraised in Oakland, Calif., Ger-wick graduated from UCD in1976 with a B.S. in biochemistry,received his Ph.D. in oceanogra-phy from Scripps in 1981, andlater did research in pharmacy atthe University of Connecticut.

Gerwick, 53, a professor of me-dicinal and natural productschemistry at the Center for Ma-rine Biotechnology and Biomedi-cine, Scripps Institution of Ocean-ography and Skaggs School ofPharmacy and PharmaceuticalSciences at the University of Cali-

fornia, San Diego, continues tocall himself “a water person.” Inhis professional capacity, he and hisstudents search the seas for naturalproducts that might become medi-cines. More generally, he adds, “Ilove to be in the water and doingthings in the water, and my happi-est place to be at the end of a longday is in the hot tub.

“We go out into the field andcollect samples,” he says, describ-ing his research and work. “Itravel with my students and we gosomewhere for two or threeweeks, oftentimes scuba diving 3or 4 times a day, surveying speciesand collecting algae of interest tous. We can’t predict what willcontain a compound of interest,so we collect small amounts ofeverything and bring them backto San Diego to test them for bio-logical activities. We’ve had somepromising compounds move topreclinical stages.”

The research “fascinates andintrigues me, and satisfies my per-sonal curiosity,” Gerwick says.Moreover, he likes teaching. “Stu-dents . . . after all. . . outlive us.”

Sometimes the research alsoscares him, particularly when itentails close encounters withsharks. “We were diving off thePacific coast of Mexico, myselfand a graduate student at thetime, Valerie Paul (currently HeadScientist of the Smithsonian Ma-rine Laboratory in Ft. Pierce, Fla).We jumped in the water, wentdown about 40 or 50 feet, andcame upon a fairly large shark. Hewas only seven feet long, but ofconsiderable girth. It was a greyreef shark—very territorial. Thisone came swimming directly to-ward us.

“Usually sharks will swim byyou, and then come back,” he

continues. “But this one was be-having very aggressively, swim-ming in tighter and tighter. I guesshe was coming in for a test nibble.I’d been diving a few months ear-lier in the Galapagos, where I’dseen lots of sharks and where it isthe custom to dive with a poker. Ihad one with me. I gave it a prettygood poke in the nose. I hit itpretty hard, and it took off. Iturned around and saw that Vale-rie had brought an underwatermovie camera and had been film-ing the entire thing.” Instead ofretreating to the safety of theirboat, he adds, “We continued tocollect algae, finished our dive,and had a good story when wecame up.”

Gerwick is married with twochildren: a son, 22, who is a grad-uate student in particle physics atthe University of Edinburgh inScotland, and a daughter, 24,who is a science writer for thecommunications office of the Or-egon Health Sciences Universityin Portland. His wife, Lena, has abackground in medicine and aPh.D. in immunology, based onresearch studying fish and theirimmune systems. With their chil-dren grown, the couple nowshares their home with Oliver, aJack Russell terrier.

They also share labs. “Uponour move to Scripps/UCSD nearlythree years ago, my wife and Imerged our laboratory effortssuch that we have several sharedgrants and research projects, aswell as separate projects,” hesays. “Our labs and offices arecontiguous, and we spend a lot oftime together at work and home.”

Marlene Cimons

Marlene Cimons is a freelance writerin Bethesda, Md.

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isolates whose taxonomies were characterizedin this way came from freshwater environ-ments.

Probing the Biosynthetic Genes that

Yield Valuable Natural Products

We and others are investigating how secondarymetabolite biosynthetic genes are distributed incyanobacteria. For example, members of ourgroup along with David Sherman and his collab-orators at the University of Michigan (UM),Ann Arbor, are beginning to isolate and charac-

terize biosynthetic gene clusters re-sponsible for making specific naturalproducts in cyanobacterial species.

Complementing this approach is onethat relies on analysis of the growingnumber of cyanobacterial genome se-quences that are or soon will be avail-able. For instance, by early 2008,sequencing of 29 cyanobacterial ge-nomes was complete, while analyses ofanother 46 such genomes were underway. The initial natural products focusis on genes encoding polyketide syn-thase (PKS) and nonribosomal peptidesynthetase (NRPS) pathways.

Together, this genetic informationreveals that the more advanced fila-mentous forms of cyanobacteria arerich in modular biosynthetic pathwaysthat integrate features from both thePKS and NRPS families. In some cases,a single cyanobacterial genome con-tains multiple pathways, making cya-nobacteria resemble some streptomy-cetes and myxobacteria in this respect.

With only a small fraction of thecompounds putatively encoded bythese biosynthetic pathways isolatedor characterized, cyanobacteria are anopportunity-rich frontier for identify-ing natural products with potentialmedical utility. To address those op-portunities, researchers are developinga growing number of innovative meth-ods with which to detect, isolate, anddetermine chemical structures and bio-logical properties of those molecularproducts. For example, we devised astrategy for using bioinformatics toguide isotope-labeled, precursor-feed-ing experiments that we call the geno-

misotopic approach.Meanwhile, developing knowledge of regula-

tory gene sequences and their protein transcrip-tional modulators also looms as critical for un-covering the full natural products capacity of

F I G U R E 2

Pie charts of a) the secondary metabolites reported from the various groups of marinecyanobacteria (Types I to V with botanical order equivalents provided), and b) themetabolites of the Type III cyanobacteria (Oscillatoriales) separated by genus (data fromthe MarinLit® database).

Figure 3 (facing page). (A) Natural products of marinecyanobacteria discussed in text with their principal biolog-ical properties in human health oriented assays. In panelsB-D, the genes, encoded proteins, and deduced catalyticdomains in the (B) curacin A biosynthetic gene cluster, (C)hectochlorin biosynthetic gene cluster, and (D) lyngbya-toxin biosynthetic gene cluster.

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F I G U R E 3

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these photosynthetic bacteria. For example, werecently began using methods similar to thosedeveloped by Rolf Muller of Saarland Universityin Saarbrucken, Germany, to study regulatorypathways in myxobacteria. Our efforts are start-ing to reveal some previously unrecognized reg-ulatory mechanisms in cyanobacteria.

Cyanobacteria from Caribbean Yield

Potent Anticancer Candidates

We launched a program in 1993, surveying ma-rine algae and cyanobacteria in Curacao in thesouthern Caribbean for bioactive natural prod-ucts. The extract of one shallow-water marinecyanobacterium, L. majuscula, was highly ac-

tive when tested against a cancerousmammalian cell line, and this findingled us to isolate a lipid that we namedcuracin A (Fig. 3).

Subsequently, we determined thatcuracin A inhibits tubulin polymeriza-tion through interactions at the colch-icine-binding site. While various scien-tists developed several routes forsynthesizing it, we began exploringhow it is made biochemically. Our ini-tial approach involved feeding stableisotope-containing precursors to L.majuscula in culture, followed by nu-clear magnetic resonance (NMR) anal-ysis of labeled curacin A. These effortsled us to collaborate closely with DavidSherman and his colleagues at UMand, subsequently, to isolate the genecluster that encodes enzymes responsi-ble for synthesizing curacin A.

We are continuing to characterizethat gene cassette as well as the en-zymes that catalyze formation of thecyclopropyl ring in curacin A (Fig. 3).This remarkable set of biochemical re-actions uses three acetate molecules(plus coenzyme A) to produce a singleenzyme-bound molecule of 3-hydroxy-3-methylglutaryl acyl carrier protein(ACP), which is the branched 6-carbonprecursor for terpenes. A series of ad-ditional reactions converts this precur-sor into a 5-carbon cyclopropyl-con-taining alkyl chain. Because several ofthese reactions involve novel biochemi-cal transformations, we are continuing adetailed mechanistic investigation that

includes X-ray crystallographic analysis of severalof these enzymes.

Similar gene cassettes are found in other ma-rine cyanobacterial natural product biosyntheticpathways. However, they yield different struc-tures such as the vinyl chloride function in thenatural product called jamaicamide A. Hence,we are investigating how highly comparablegene pathways and enzymes lead to such diver-gent functional groups.

Additional Promising Natural Products

from Marine Cyanobacteria

An organic extract of a small tuft of L. majus-cula that was collected in 1996 near the eastern

F I G U R E 4

Imaging of the sponge-cyanobacterial cell assemblage Dysidea herbacea with its cya-nobacterial symbiont Oscillatoria spongelae by light microscopy (A) and epifluorescencemicroscopy (B) showing short rods of cyanobacterial cells surrounded by irregular spongecells. (C) Control epifluorescence micrograph showing location of cyanobacterial cells. (D)The same micrograph as in panel C probed with the sponge-derived halogenase geneprobe (CARD-FISH) showing that this unique halogenase, responsible for creatingtrichloromethyl groups as in barbamide, is exclusively located in the cyanobacterial cells.(Reprinted from P. M. Flatt, J. T. Gautschi, R. W. Thacker, M. Musafija-Girt, P. Crews, andW. H. Gerwick, Marine Biol. 147:761-774, 2005, with kind permission of Springer Scienceand Business Media.)

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shore of Jamaica yielded powerful anticancercell activity. Because the sample extracts wereinsufficient for chemical studies, viable materialfrom a single filament was clonally expanded,and it yielded two exceptionally interesting sec-ondary metabolite classes, designated the jamai-camides and hectochlorin (Fig. 3). Hectochlorinproved to be the agent responsible for that ini-tial, powerful cancer cell toxicity, whereas thejamaicamides displayed moderate neurotoxicproperties.

Jamaicamide A incorporates three distinctfunctional groups whose biosynthetic originsremain poorly understood—namely, a vinylchloride appendage on a polyketide, a terminalacetylenic bromide, and a pyrrolidone ring.While we deduced the fundamental buildingblocks of jamaicamide A from a series of exper-iments involving the use of stable isotope-la-beled precursors, figuring out the biosyntheticchemistry that leads to its assembly required usto take a molecular genetics-based approach.Thus, we developed a cloning strategy thatyielded a 58-kb gene cluster, which encodes theproteins that catalyze jamaicamide biosynthesis.We are continuing to investigate the steps thatform the distinctive vinyl chloride functionalityof this terpene variant.

We also are investigating hectochlorin bio-synthesis, focusing on the origin of the gem-dichloro functionality within its polyketide andon the basis for the two different regiochemicalincorporation patterns of 2,3-dihydroxyisova-leric acid (Fig. 3). We find high sequence homol-ogy of hctB with barB1 and barB2, which aretwo marine examples of radical halogenasesthat we discovered within the barbamide bio-synthetic gene cluster that specifically haloge-nate a methyl group. The hctB gene encodes aradical halogenase that catalyzes chlorination ofa carbon atom other than a methyl group. Weare collaborating with Christopher Walsh atHarvard University and his collaborators, seek-ing to explain the structural basis for this alter-nate regiochemical selectivity.

Drifting Tufts of Cyanobacteria Can

Produce Biologically Active Agents

Sometimes waves and currents dislodge cya-nobacterial filaments. Those filaments, in turn,excrete a slimy mucilage that entraps bubbles ofphotosynthetically derived oxygen, floating the

filaments to the surface where they continue togrow with other plankton. Sometimes thesefloating assemblages drift as large cyanobacte-rial tufts, and are common in places such asHawaii and northeastern Australia.

Some dislodged and drifting strains of cya-nobacteria produce extremely potent dermato-toxins that cause harm in near-shore environ-ments, especially when they are being used bylocal swimmers or water-loving tourists. In Ha-waii, this phenomenon gives rise to sporadicbouts of swimmer’s itch, whose immediatecause is an alkaloid, that Richard Moore of theUniversity of Hawaii described in 1979 andnamed lyngbyatoxin A (Fig. 3). It contains anumber of noteworthy structural features, in-cluding a lactam ring, a reduced phenylalaninecarboxyl group, and a reverse-prenylated in-dole. Some strains of L. majuscula produce an-other type of dermatotoxin, called aplysiatoxin.

Because of the potential public health benefitsfrom understanding better how these environ-mental toxins are produced—and also out ofpure curiosity—we began characterizing biosyn-thetic gene clusters in toxin-producing cya-nobacteria collected near Hawaii. Before begin-ning, we recognized that, early in the process,the NRPS involved in producing lyngbyatoxinlikely forms a peptide bond between valine andtryptophan. Moreover, a biochemical reductionreaction likely releases that dipeptide from theNRPS complex. This hypothesis was recentlyconfirmed by Walsh and his collaborators atHarvard, who characterized the reductive mech-anism for releasing the lyngbyatoxin dipeptidefrom its NRPS complex.

With those biochemical steps in mind, wedevised a cloning strategy to isolate the biosyn-thetic gene cluster. From a bioinformatic analy-sis of the cluster, we postulate that a cytochromeP450 activates the phenyl ring of tryptophansuch that the lactam ring of lyngbyatoxin can beformed. Next, a reverse prenylation at the oppo-site side of this ring occurs, possibly first at theindole nitrogen atom, and then through aClaisen-type rearrangement, moving to the finalC-7 location.

Many Natural Products Derive from

Invertebrate-Associated Microorganisms

Microorganisms produce many of the naturalproducts that chemists isolate from marine in-

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vertebrate sources. More generally, marine in-vertebrate species are remarkable for how ex-tensively they associate with marine microbiota.For instance, some sponges consist of 40% bac-teria by weight! These and other marine inver-tebrates harbor diverse microorganisms, some-times benefitting through rich suites of microbiallyproduced chemicals that protect these spongesor other hosts against predators or pathogens.

Meanwhile, however, microbiologists havehad little success trying to cultivate these inver-tebrate-associated microorganisms. Hence, littleis yet known about precisely which microorgan-isms are producing the secondary metabolites.To date, most insights come from non-culture-based methods, including cell separations, imag-ing with immunogold reagents, direct imagingthat depends on spectral features of the moleculeunder study, catalyzed reporter deposition fluo-rescence in situ hybridization (CARD-FISH) tolocalize biosynthetic genes within particular celltypes, and mass spectrometric imaging (MSI)(Fig. 4).

Although MSI rapidly and directly determineswhere natural products are located in complexassemblages, even this type of informationsometimes can prove misleading. Thus, for ex-ample, one cell type might produce and thensecrete a particular compound that is then selec-tively absorbed by a second cell type. This sce-

nario apparently explains earlier confusion overthe origins of cyclic peptides that are associatedwith the tunicate Lissoclinum patella. Research-ers thought that this tunicate produces cyclicpeptides in its own tissues. However, subse-quent studies involving gene cloning, expres-sion, and genomic sequencing indicate that sym-biotic cyanobacteria actually produce thesepeptides. It is possible that through the processof metabolite excretion from this microbialsource that they can accumulate and be detectedin the tunicate tissues.

These giant marine cyanobacteria are indeedan exciting area for microbial research. Theirstructurally novel secondary metabolites arisefrom unique biochemical pathways and machin-ery that are equally worthy of study. Comple-menting this structural and biosynthetic nov-elty, the activities of some of these metabolitesare exerted through new mechanisms that revealinnovative pathways with pharmacological po-tential. From the phylogenetic relationship ofthese organisms, the evolution of their naturalproduct pathways, and the structures and activ-ities of their metabolites, these organisms havemuch to teach us, much to provide in terms ofuseful products and bio-chemical tools, and ul-timately, much to inspire us with through inte-gration of natural products chemistry and mi-crobiology.

SUGGESTED READING

Corre, C., and G. L. Challis. 2007. Heavy tools for genome mining. Chem. Biol. 14:7–9.Donia, M. S., B. J. Hathaway, S. Sudek, M. G. Haygood, M. J. Rosovitz, J. Ravel, and E. W. Schmidt. 2006. Naturalcombinatorial peptide libraries in cyanobacterial symbionts of marine ascidians. Nature Chem. Biol. 2:729–735.Edwards, D. J., and W. H. Gerwick. 2004. Lyngbyatoxin biosynthesis: sequence of biosynthetic gene cluster and identificationof a novel aromatic prenyltransferase. J. Am. Chem. Soc. 126:11432–11433.Flatt, P. M., J. T. Gautschi, R. W. Thacker, M. Musafija-Girt, P. Crews, and W. H. Gerwick. 2005. Identification of thecellular site of polychlorinated peptide biosynthesis in the marine sponge Dysidea (Lamellodysidea) herbacea and symbioticcyanobacterium Oscillatoria spongeliae by CARD-FISH analysis. Marine Biol. 147:761–774.Hildebrand, M., L. E. Waggoner, G. E. Lim, K. H. Sharp, C. P. Ridley, and M. G. Haygood. 2004. Approaches to identify,clone, and express symbiont bioactive metabolite genes. Natural Product Rep. 21:122–142.Newman, D. J., and G. M. Cragg. 2007. Natural products as sources of new drugs over the last 25 years. J. Natural Products70:461–477.Rachid, S., K. Gerth, I. Kochems, and R. Muller. 2007. Deciphering regulatory mechanisms for secondary metaboliteproduction in the myxobacterium Sorangium cellulosum So ce56. Mol. Microbiol. 63:1783–1796.Ramaswamy, A. V., C. M. Sorrels, and W. H. Gerwick. 2007. Cloning and biochemical characterization of the hectochlorinbiosynthetic gene cluster from the marine cyanobacterium Lyngbya majuscula. J. Natural Products 70:1977–1986.Simmons, T. L., R. C. Coates, B. R. Clark, N. Engene, D. Gonzalez, E. Esquenazi, P. C. Dorrestein, and W. H. Gerwick. 2008.Biosynthetic origin of natural products isolated from marine microorganism-invertebrate assemblages. Proc. Natl. Acad. Sci.USA 105:4587–4594.Simmons, T. L., and W. H. Gerwick. 2008. Anticancer drugs of marine origin, p. 431–452. In Oceans and human health.Elsevier, Academic Press, Burlington, Mass.Van Wagoner, R., A. K. Drummond, and J. L. C. Wright. 2007. Biogenetic diversity of cyanobacterial metabolites. Adv. Appl.Microbiol. 61:89–217.

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