BIOLOGY & MEDICINE_Viruses

Tiny giants

Viruses are usually incredibly small, but some deviate from the norm and reach sizes greater than that of a bacterial cell. Matthias Fischer from the Max Planck Institute for Medical Research in Heidelberg is one of a small number of scientists working on giant viruses of this kind.

TEXT STEFANIE REINBERGER Photo: Wolfram Scheible

58 MaxPlanckResearch 3 | 19 n the laboratory of Matthias Fischer Although they look like nothing more As giant viruses are about at the Max Planck Institute in Hei- than vials of water to the naked eye, the the same size as bacteria, delberg, vials containing water samples are actually teeming with it is almost impossible to purify them by filtration samples are lined up against one life, which only becomes visible when only. However, as viruses another, each containing a whole viewed through a microscope: countless and bacteria have different I world of aquatic single-celled organ­ tiny dots are scurrying back and forth. densities, they form layers isms and viruses. The labels reveal the “The smaller ones are bacteria, which when spun in an ultracen- trifuge. Scientists can then origins of the samples: Guenzburg, are devoured by larger cells that have a extract the viral band using Kiel, but also more exotic locations nucleus. These so-called protists are the a syringe and needle. such as Tallinn or the British Virgin reason we created the collection in the Islands. “The collection is the result of first place,” Fischer explains. Indeed, many years of work,” the microbiolo- these protists are susceptible to attack

Photo: Wolfram Scheible gist explains. by giant viruses, which are Fischer‘s

3 | 19 MaxPlanckResearch 59 60 protists. for source food a as serve turn in which bacteria, of growth the stimulate vials the in grains Wheat environments. different from single- with teems vial each microcosms: Global BIOLOGY &MEDICINE _Viruses MaxPlanckResearch 3 MaxPlanckResearch ­ce lled organisms and viruses isolated isolated viruses and organisms lled | 19

they haveonethingincommon: they their genomes,structuresand shapes, virus particles differ widely in terms of est soilsandpermafrost.Although the sewage treatmentplants,tree barks,for been discoveredinoceans, saltlakes, Many othergiantviruseshavesince virus that pretends to be a bacterium. virus, frommimickingmicrobe–a giant anew, moresuitablename:mimi about adozengenes. influenzavirusesandHIVcarry ison, encoded some1,000genes.Bycompar largest knownvirusgenometodateand lion base pairs, it was twice as big as the also hadahugegenome:with1.2mil- nanometers. Moreover, foravirus,it some bacteria, with a diameter of 750 virus –onethatwasevenbiggerthan that theywereactuallydealingwitha at Aix-MarseilleUniversityrealized named itBradfordcoccus. a bacterium, the researchers initially Bradford (UK).Assumingthevirustobe cuit ofanindustrialcoolingtowerin in anamoebafoundthewatercir- was discoveredbyresearchers in1992 The first example of these viral giants initially mistookthemforbacteria. bigger than other viruses that scientists virus world.Indeed,theyaresomuch standards, buttheyarethegiantsof These virusesmaybetinybyhuman AS ABACTERIUM A VIRUSDISGUISED that canactashostsforgiantviruses. dian Ocean for single-celled organisms of theDanube,BalticSeaorIn- leagues, he therefore screens the waters main interest.Together withhiscol The Frenchresearchers gavethe It was not until 2003 that researchers ­ - ­ -

carried outusingsmall-porefiltersthat ters. Virus experimentsweretherefore mum diameter of 200 to 300 nanome- searchers wasthat viruses hadamaxi- betweencellsandviruses. boundary blur thepreviouslyfirmlyestablished methods is thus rather small. Often,the ideas andexchangeknowledge and whom MatthiasFischercan discuss viruses. Thecircle ofcolleagueswith many of which are also hosts of giant conducted intothemajority of protists, and toxoplasmosis. sponsible formalaria,sleepingsickness nificance, suchasthepathogensre species areofmedicaloreconomicsig- the world‘sfoodwebs,butonlyfew isms. Protistsplayanimportantrolein themselves, butalsooftheirhostorgan exotic naturenotonlyoftheviruses who studythem.Thismaybeduetothe are stillonlyafewscientistsworldwide greater attentioninrecentyears,there Although giantviruseshavereceived OF GIANTVIRUSES A GROWING ARMY thias Fischer. to discoversomething new,” says Mat- our assumptionsifwegenuinelywant teach usisthereforetoalwaysquestion tions. Oneofthelessonsgiantviruses long is caused by our own preconcep- they remainedundiscoveredforso overlooked. “Ultimately, thefactthat organisms –andthereforequitesimply with bacteriaandothersingle-celled tered outofthesamplematerialalong a result,giantvirusesweresimplyfil the commonsizedefinitiontopass.As only allowedrepresentativesmeeting So far, hardlyanyresearch hasbeen The unwrittenruleamongvirusre- - ­ -

Photo: Wolfram Scheible Matthias Fischer uses pulsed-field gel electrophoresis to separate the genomes of giant viruses for further analysis.

standard laboratory techniques don’t work for giant viruses or their hosts. In that case, Fischer has to spend a great deal of time finding alternatives – or new techniques altogether. “On the up- side, however, there’s a good chance we’ll stumble across something new and unexpected in each of our projects,”­ he says. You could describe the 42-year-old microbiologist as a modern-day discov- erer. He has a sparkle in his eye as he talks about the unsolved mysteries of giant viruses, and you can sense his eagerness to crack as many of them as possible. And there is no shortage of riddles surrounding these giants. One question is how many different types of giant viruses remain to be discov- ered, another one is their unresolved evolutionary origin and why they are so big in the first place. For some time, the question as to their origin has divided the small com- munity of researchers into two camps. Some believe the giant viruses emerged from cells that gradually lost most of their genetic material. With the remain- ing genes, the diminished cells were no longer able to multiply on their own and became dependent on the machin- ery of other cells. Giant viruses may therefore be the remnants of an other- wise extinct domain of life, alongside bacteria, archaea and eukaryotes (cells with a nucleus). “However, our re- search findings – and those of col- leagues – suggest that giant viruses

Photo: Wolfram Scheible Photo: Wolfram Scheible rather evolved from smaller viruses,

3 | 19 MaxPlanckResearch 61 62 (right). virophage the reactivates infection CroV until generations many for persist can it where material, genetic host’s the into itself incorporates it but flagellate, the in multiply cannot alone Mavirus (center). Mavirus of action the by infection CroV from protected are cells host other dies, ultimately cell infected the Although replicates. Mavirus instead, and particles infectious new produce to unable is virus giant the Mavirus, virophage the with infected simultaneously is flagellate the If (left). cycle infection the of end the at it kills roenbergensis flagellate the host, its infects CroV virus giant the when viruses: two host, One Cell lysisreleasesCroVparticles MaxPlanckResearch 3 MaxPlanckResearch Virophage Mavirus Giant virusCroV Flagellate , it multiples inside the cell and and cell the inside multiples it , Cafeteria roenbergensis | 19 CroV infection Cafeteria Cafeteria

was absolutely fascinated because, until was absolutelyfascinatedbecause, until the diversityofvirusesin ocean.“I he cameacrossascientific articleon inBayreuth,when gree inbiochemistry was sparkedtowardstheend ofhisde- Fischer’s personalinterestinviruses FOR RESEARCHERS NEW TERRITORY cating theirown,”saysFischer. while also dupli- organisms other from continually incorporatingnewgenes Cell lysisreleasesMavirusparticles Co-infection and Mavirus with CroV the expenseofCroV Mavirus replicatesat

Exposure toMavirus new virus,Cafeteriaroenbergensis virus dent fromGermanysetout tostudya perseverance. Theyoungdoctoral stu- in Vancouver demandedagreatdealof further scientificcareer. The first years stayed forsixyears.” doctoral position,bookedaflightand Suttle, inVancouver. “Iappliedfora tacted theauthorofarticle,Curtis particles,” Fischerrecalls.Hethencon- seawater contains several billion virus literof then, Ihadnoideathatevery Cell lysisreleasesparticlesofCroVandMavirus This work set thecourseforFischer’s CroV infection into thehostgenome Mavirus integrates Mavirus CroV reactivates

Graphic: M. Fischer/MPI for Medical Research Graphic: M. Fischer/MPI for Medical Research

Photo: M. Fischer/MPI for Medical Research cause Mavirusactsasaparasite ofCroV, harm thehost.Oncontrary, be tanously infectedwithCroV, itdoesnot tiplies withinahostcellthat issimul- name ofMavirus.AsMavirus onlymul er virus–a CroV itselfisinfectedbyamuchsmall entist tothisday. Fischerrealizedthat ery ultimately led to animportant discov further studies. But this circumstance ten so low that he couldn’t use it for from hisinfectionexperimentswasof- problems todealwith:thevirusyield letters ofthevirusgenome. him tofinallyreadmorethan600,000 sequencing technologieswouldallow teria. A few years later the arrival of new DNA wasapparentlytoxicforthebac- experiment failed repeatedly. The viral decipher them pieceby piece – but this it intobacteriainordertomultiplyand viral genomebyincorporatingpiecesof on hisside:heplannedtodecodethe tially, however, luckdidn’t appeartobe It wasFischer’s jobtochangethat.Ini- years but hadn’t investigated it further. about thegiantvirusforanumberof At thetime,researchers hadknown TOXIC DNA on theplanet. is oneofthemostnumerouspredators zooplankton that feeds on bacteria and bergensis, awidespreadspeciesofmarine organism bythenameofCafeteriaroen­ viruses. CroVinfectsasingle-celled long tothesamefamilyasmimi (CroV), whichlaterturnedouttobe , whichcontinuestooccupythesci- In themeantime,Fischerhadother » so-called virophage by the in a mutually beneficial symbiosis with their hosts. theirsymbiosis with beneficial mutually a in exist organisms;viruses rather, host some their of parasites therefore are viruses all Not giantvirus. the from flagellates the protecting system, defense likea operates virussmall The - - ­ ­ ­ - cell. Sofar, thisenzymehad onlybeen genetic materialintothatof thehost known asanintegrasetoinsert itsown the smallvirususesaprotein discovery: based researchers made an astonishing detail. Intheprocess,Heidelberg- infection cycleofMavirusingreater team, thescientistbegantostudy this unusualalliance.Together withhis C. roenbergensis andMavirus. ally beneficialsymbiosisexistsbetween parasites oftheirhosts;rather, amutu- a whole.”Notallvirusesaretherefore ofthepopulationas ensure thesurvival that singleindividualsaresacrificedto it seemstobeawidespreadprinciple says Fischer. “Amongmicroorganisms, same species against the giant virus,” cell inordertoprotectmembersofthe altruistic defensesystemforthehost “The smallvirusthereforeactslikean PROTECTKIN YOUR with CroVandthepopulationsurvives. bors: theyaresparedfrominfection it matters for its not yet infected neigh- make adifferencefortheinfectedcell, CroV particles.Althoughthisdoesn’t new Mavirusesparticlesinsteadof the smallerMavirus,cellreleases However, whenCroVisparasitizedby and theviraloffspringarereleased. sufficient quantities, the host cell bursts is a lytic virus: once it has multiplied in dies upon infection with CroV, which ly. Thissingle-celledorganismtypically roenbergensis viruses andtherebyhelpsitshostC. it preventstheproductionofnewgiant Fischer wantedtoknowmoreabout to survive albeitindirect- to survive ­ the Mavirus virophage (red, false color). false (red, virophage Mavirus the and (blue) CroV virus giant the of Particles 100 BIOLOGY &MEDICINE _Viruses nm

3 | MaxPlanckResearch 19MaxPlanckResearch 63 BIOLOGY & MEDICINE_Viruses Setting signs!

Ingo Barth is a senior researcher, and deaf. In daily communication, tech terms such as »tunnel ionization« are giving him hard times as a scientist, because often matching known to occur in retroviruses such as either because the host needs to adapt ever, this principle applies to all living elements are lacking in German sign lan- HIV and related jumping genes – or because it assimilates some of its par- things, as the human genome is also guage. The Foundation supports his project known as transposons – but not in DNA asite’s genes and capabilities. littered with parasitic elements, such as viruses such as virophages. Accordingly, parasites provide their transposons or remnants of viruses. In at the Max Planck Institute for Microstruc- For Mavirus, this appears to be a hosts with new genetic information – other words, viruses and other parasites ture Physics to develop a German STEM good strategy: if the virus does not rely and therefore new traits. This so-called played a substantial role in human evo- sign language dictionary to enhance equal on CroV for the incorporation of its ge- horizontal gene transfer by viruses lution, too. “This is an incredibly excit- netic material, it can be passed from one plays a central role in the evolution of ing realization,” says Fischer. “Although opportunities and diversity in our research cell generation to the next even without microorganisms. our research into giant viruses and environment. the giant virus. “This ensures that It may also have been the driving virophages seems exotic at first glance, Mavirus is already present when a suit- force for the evolution of giant viruses it provides us with deep insights into able giant virus infects the host cell,” from smaller viruses. Ultimately, how­ universal processes of evolution.” says Fischer. For multiplication, howev- er the virophage depends on the giant virus and its replication machinery.

ANZEIGE

PARASITES SUMMARY ARE OMNIPRESENT l Giant viruses can contain over 1,000 genes and presumably evolved from smaller viruses, adopting their hosts’ genetic material as well as duplicating genes of But does the behavior Fischer observed their own. in the lab really play a role in natural l Giant viruses are primarily parasites of single-celled eukaryotes (protists). However, they are sometimes infected with parasites themselves: virophages use ecosystems? To answer this question, the the giants in order to multiply within the host cell. researcher analyzes C. roenbergensis ge- l With the ability to incorporate their own genes into foreign genetic material, nomes from various water samples. So some viruses modify their hosts’ genome, thus acting as a driving force of evolution. far, he found Mavirus in all of them. And that’s not all: other virophages are also present in the single-celled organism’s genetic material. Some of the incorpo- GLOSSARY The Max Planck Foundation has sup ported rated DNA is very similar to that of Giant viruses: These unusually complex viruses primarily infect single-celled eukaryotes. the Max Planck Society for more than ten Mavirus, while others represent hitherto-­ The largest specimens are approx. 2 micrometers long and contain over 1,000 genes, years by providing targeted funding for top- unknown virophages. Moreover, some of whose functions render giant viruses independent of the host cell in many biochemical processes. The particles (capsids) of giant viruses are made up of multiple layers con­ level innovative and cutting-edge research the virophage genomes are interrupted sisting of hundreds of proteins, with special exit portals for the packaged virus genome. by transposons. In other words, even Despite their global distribution and diversity, the ecological role of giant viruses is at the more than 80 institutes, enabling virophages – which are parasites of par- still unknown. breakthroughs in frontier science. As a asites – have their own parasites. Transposons: DNA fragments that can alter their position in the genome. They are also Step by step, Fischer and his team referred to as “jumping genes,” although they often contain multiple genes. Transposons patron, you can make a crucial difference are decoding an interwoven chain of that multiply via an RNA intermediate are known as retrotransposons. These are probably also the origin of retroviruses, which incorporate themselves into the host’s genetic by creating additional scope to keep this single-celled organisms, viruses and material but can also infect other hosts by forming infectious virus particles. research ahead of the curve in the inter- parasitic DNA elements. For him, this Virophages: So far, only a handful of virophages are known. In addition to Mavirus, national scientific competition. Join us! is the crucial point. “Parasites that in- examples include Sputnik and Zamilon, which are parasites of mimiviruses. Virophages fect other parasites are a widespread can only multiply in host cells that are infected with a suitable giant virus at the same phenomenon in nature,” says Fischer. time. They replicate at the expense of giant viruses by hijacking some of their enzymes. “They are a key driver of evolutionary In other words, virophages are parasites of giant viruses and therefore beneficial to their host cells. processes.” The reason for this is that Max-Planck-Foundation every parasite leaves traces in its host, Deutsche Bank www.maxplanckfoundation.org IBAN DE46 7007 0010 0195 3306 00 64 MaxPlanckResearch 3 | 19 Setting signs!

Ingo Barth is a senior researcher, and deaf. In daily communication, tech terms such as »tunnel ionization« are giving him hard times as a scientist, because often matching elements are lacking in German sign lan- guage. The Foundation supports his project at the Max Planck Institute for Microstruc- ture Physics to develop a German STEM sign language dictionary to enhance equal opportunities and diversity in our research environment. ANZEIGE

The Max Planck Foundation has sup ported the Max Planck Society for more than ten years by providing targeted funding for top- level innovative and cutting-edge research at the more than 80 institutes, enabling breakthroughs in frontier science. As a patron, you can make a crucial difference by creating additional scope to keep this research ahead of the curve in the inter- national scientific competition. Join us!

Max-Planck-Foundation Deutsche Bank www.maxplanckfoundation.org IBAN DE46 7007 0010 0195 3306 00