Bacteriophage- types, structure and morphology of t4 phage, morphogenesis

Escherichia virus T4 is a species of bacteriophages that infect Escherichia coli bacteria. It is a member of virus subfamily Tevenvirinae (not to be confused with T-even bacteriophages, which is an alternate name of the species). T4 is capable of undergoing only a lytic lifecycle and not the lysogenic lifecycle.

1. BACTERIOPHAGE:
Dr. Dinesh C. Sharma,
Associate Professor & Head
Deptof Zoology
Km. MayawatiGovt. Girls P.G. College, Badalpur, Gb nagar

2. Abacteriophage,alsoknowninformallyasaphage,isavirusthat
infectsandreplicateswithinbacteriaandarchaea.Thetermwas
derivedfrom"bacteria"andtheGreek,meaning "todevour".
Bacteriophages are composed
ofproteinsthatencapsulateaDNAorRNAgenome,andmayhave
structuresthatareeithersimpleorelaborate.Theirgenomes may
encodeasfewasfourgenes(e.g.MS2)andasmanyashundreds
ofgenes.Phagesreplicatewithinthebacteriumfollowingthe
injectionoftheirgenomeintoitscytoplasm.
Bacteriophages areubiquitousviruses,foundwhereverbacteria
exist.Itisestimatedtherearemorethan10
31
bacteriophagesonthe
planet,morethaneveryotherorganismonEarth,includingbacteria,
combined andupto70%ofmarinebacteriamaybeinfectedby
phages.
Phageshavebeenusedsincethelate19thcenturyasanalternative
toantibioticsintheformerSovietUnionandCentralEurope,aswell
asinFrance.Theyareseenasapossibletherapyagainstmulti-drug-
resistantstrainsofmanybacteria.

3. Phylum: incertaesedis
Class: incertaesedis
Order: Caudovirales
Family: Myoviridae
Genus: Tequatrovirus
Species: Escherichia virus T4
Virus classification
EscherichiavirusT4isaspeciesofbacteriophages that
infectEscherichiacolibacteria.Itisamember ofvirus
subfamilyTevenvirinae(nottobeconfused withT-even
bacteriophages,whichisanalternatenameofthespecies).T4is
capableofundergoingonlyalyticlifecycleandnotthelysogenic
lifecycle.

4. TheT4-typebacteriophages are
ubiquitouslydistributedinnature
andoccupyenvironmentalniches
rangingfrommammalian gutto
soil,sewage,andoceans.
Morethan130suchvirusesthat
show similar morphological
featuresasphageT4havebeen
described; from the T4
superfamily~1400majorcapsid
proteinsequences havebeen
correlatedtoits3Dstructure.
The features include large
elongated (prolate) head,
contractiletail,andacomplex
baseplatewithsixlong,kinked
tailfibersradiallyemanatingfrom
it.

6. StructureofphageT4capsid
Theoverallarchitectureofthephage
T4headdeterminedbynegativestain
electronmicroscopyoftheprocapsid,
capsid,andpolyhead,includingthe
positionsofthedispensableHocand
Socproteins,hasbasicallynotchanged
asaresultofcryo-electronmicroscopic
structuredeterminationofisometric
capsids.However,thedimensionsof
thephageT4capsidanditsinferred
proteincopynumbershavebeen
slightlyalteredonthebasisofthe
higherresolutioncryo-electron
microscopystructure.Thewidthand
lengthoftheelongatedprolate
icosahedronareT
end=13laevoand
T
mid=20(86nmwideand120nm
long),andthecopynumbersofgp23,
HocandSocare960,155,and870,
respectively
StructureofthebacteriophageT4head.A)Cryo-EMreconstructionof
phageT4capsid;thesquareblockshowsenlargedviewshowinggp23
(yellowsubunits),gp24(purplesubunits),Hoc(redsubunits)andSoc
(whitesubunits);B)StructureofRB49Soc;C)Structuralmodelshowing
onegp23hexamer(blue)surroundedbysixSoctrimers(red).Neighboring
gp23hexamersareshowningreen,blackandmagenta;D)Structureof
gp24[6];E)Structuralmodelofgp24pentamericvertex.

7. The bacteriophage T4 capsid is an elongated icosahedron, 120 nm long and 86 nm wide, and is built with three
essential proteins; gp23*, which forms the hexagonal capsid lattice, gp24*, which forms pentamers at eleven of
the twelve vertices, and gp20, which forms the unique dodecameric portal vertex through which DNA enters
during packaging and exits during infection.
The capsid also contains two non-essential outer capsid proteins, Hoc and Soc, which decorate the capsid surface.

8. Displayoncapsid
Inadditiontotheessentialcapsidproteins,gp23,gp24,and
gp20,theT4capsidisdecoratedwith
twonon-essentialoutercapsidproteins:
Hoc(highlyantigenicoutercapsidprotein),adumbbell
shapedmonomeratthecenterofeachgp23hexon,upto155
copiespercapsid(39kDa;redsubunits);and
Soc(smalloutercapsidprotein),arod-shapedmoleculethat
bindsbetweengp23hexons,upto870copiespercapsid(9kDa;
whitesubunits).BothHocandSocaredispensable,andbindto
thecapsidafterthecompletionofcapsidassembly.Null(amber
ordeletion)mutationsineitherorboththegenesdonotaffect
phageproduction,viability,orinfectivity.
Aninvitrodisplaysystemhasbeendevelopedtakingadvantageofthe
highaffinityinteractionsbetweenHocorSocandthecapsid
Inthissystem,thepathogenantigenfusedtoHocorSocwithahexa-
histidinetagwasoverexpressedinE.coliandpurified
In vitro display of antigens on bacteriophage T4 capsid. Schematic representation of the T4 capsid decorated with large
antigens, PA (83 kDa) and LF (89 kDa), or hetero-oligomeric anthrax toxin complexes through either Hoc or Soc binding . The
insets show electron micrographs of T4 phage with the anthrax toxin complexes displayed through Soc (top) or Hoc (bottom).

9. Structure of the packaged
components ofthephageT4head
PackagedphageT4DNAsharesa
numberofgeneralfeatureswith
othertaileddsDNAphages:2.5nm
side to side packing of
predominantly B-formduplexDNA
condensed to~500mg/ml.The
discontinuouspatternofDNAsuch
asintheicosahedral-bendor
spiral-foldmodels.
TheinternalproteinI*(IPI*)of
phageT4isinjectedtoprotectthe
DNA from a estriction
endonuclease ofapathogenicE.
colithatdigestsglucosylated
hydroxymethylcytosine DNAofT-
evenphages
Models of packaged DNA structure.a)T4 DNA is packed
longitudinally to the head-tail axis , unlike the transverse
packaging in T7 capsids (b). Other models shown include spiral
fold(c), liquid-crystal(d), and icosahedral-bend(e). Both
packaged T4 DNA ends are located in the portal .

10. DNApackaging
Twononstructuralterminaseproteins,gp16(18kDa)andgp17(70
kDa),linkheadassemblyandgenomeprocessing.
Theseproteinsarethoughttoformahetero-oligomericcomplex,
which recognizes theconcatemeric DNA andmakes an
endonucleolyticcut(hencethename"terminase").Theterminase-
DNAcomplexdocksontheproheadthroughgp17interactionswith
thespecialportalvertexformedbythedodecameric gp20,thus
assemblingaDNApackagingmachine.
TheT4virus'sdouble-stranded DNA genome isabout
169kbplongandencodes 289proteins.TheT4genome
isterminallyredundantandisfirstreplicatedasaunit,then
severalgenomic unitsarerecombined end-to-endtoform
aconcatemer.When packaged, theconcatemer iscutat
unspecificpositionsofthesamelength,leadingtoseveral
genomesthatrepresentcircularpermutationsoftheoriginal.The
T4genomebearseukaryote-likeintronsequences

11. Replication
Bacteriophagesmayhavealyticcycleoralysogeniccycle.
WithlyticphagessuchastheT4phage,bacterialcellsarebrokenopen(lysed)and
destroyedafterimmediatereplicationofthevirion.Assoonasthecellisdestroyed,the
phageprogenycanfindnewhoststoinfect.Lyticphagesaremoresuitableforphage
therapy.Somelyticphagesundergoaphenomenon knownaslysisinhibition,where
completedphageprogenywillnotimmediatelylyseoutofthecellifextracellularphage
concentrationsarehigh.
Incontrast,thelysogeniccycledoesnotresultinimmediatelysingofthehostcell.
Thosephagesabletoundergolysogenyareknownastemperatephages.Theirviral
genomewillintegratewithhostDNAandreplicatealongwithit,relativelyharmlessly,or
mayevenbecome establishedasaplasmid.Thevirusremainsdormantuntilhost
conditions deteriorate, perhaps due todepletion ofnutrients, then,
theendogenousphages(knownasprophages)becomeactive.Atthispointtheyinitiate
thereproductivecycle,resultinginlysisofthehostcell.Asthelysogeniccycleallows
thehostcelltocontinuetosurviveandreproduce,thevirusisreplicatedinalloffspring
ofthecell.Anexampleofabacteriophageknowntofollowthelysogeniccycleandthe
lyticcycleisthephagelambdaofE.coli.
T4iscapableofundergoingonlyalyticlifecycleandnot
thelysogeniclifecycle.

13. Infectionprocess
TheT4virusinitiatesanEscherichiacoliinfectionbybinding
OmpCporinproteinsandlipopolysaccharide(LPS)onthesurface
ofE.colicellswithitslongtailfibers(LTF).
ArecognitionsignalissentthroughtheLTFstothebaseplate.
Thisunravelstheshorttailfibers(STF)thatbindirreversiblyto
theE.colicellsurface.Thebaseplatechangesconformationand
thetailsheathcontracts,causingGP5attheendofthetailtubeto
puncturetheoutermembrane ofthecell.
ThelysozymedomainofGP5isactivatedanddegradesthe
periplasmicpeptidoglycan layer.Theremainingpartofthe
membrane isdegradedandthenDNAfromtheheadofthevirus
cantravelthroughthetailtubeandentertheE.colicell.
Morphogenesis

14. Reproduction
Thelyticlifecycle(fromenteringabacteriumtoitsdestruction)
takesapproximately30minutes(at37°C)andconsistsof:
•Adsorptionandpenetration(startingimmediately)
•Arrestofhostgeneexpression(startingimmediately)
•Enzymesynthesis(startingafter5minutes)
•DNAreplication(startingafter10minutes)
•Formationofnewvirusparticles(startingafter12minutes)
Afterthelifecycleiscomplete,thehostcellburstsopenand
ejectsthenewlybuiltvirusesintotheenvironment,destroying
thehostcell.T4hasaburstsizeofapproximately100-150viral
particlesperinfectedhost.TheseEscherichiavirusesinfecta
hostcellwiththeirinformationandthenblowupthehostcell,
therebypropagatingthemselves.

15. Attachment and penetration
Bacterialcellsareprotectedbyacellwallofpolysaccharides,whichareimportant
virulencefactorsprotectingbacterialcellsagainstbothimmune hostdefenses
andantibiotics.Toenterahostcell,bacteriophagesattachtospecificreceptorsonthe
surfaceofbacteria,includinglipopolysaccharides, teichoicacids,proteins,or
evenflagella.Thisspecificitymeansabacteriophagecaninfectonlycertainbacteria
bearingreceptorstowhichtheycanbind,whichinturn,determinesthephage'shost
range.Polysaccharide-degrading enzymes, likeendolysinsarevirion-associated
proteinstoenzymaticallydegradethecapsularouterlayeroftheirhosts,attheinitial
stepofatightlyprogrammed phageinfectionprocess.Hostgrowthconditionsalso
influencetheabilityofthephagetoattachandinvadethem.Asphagevirionsdonot
moveindependently,theymustrelyonrandomencounterswiththecorrectreceptors
wheninsolution,suchasblood,lymphaticcirculation,irrigation,soilwater,etc.
Myovirusbacteriophages useahypodermicsyringe-likemotiontoinjecttheirgenetic
materialintothecell.Aftercontactingtheappropriatereceptor,thetailfibersflexto
bringthebaseplateclosertothesurfaceofthecell.Thisisknownasreversible
binding.Onceattachedcompletely,irreversiblebindingisinitiatedandthetail
contracts,possiblywiththehelpofATP,presentinthetail,injectinggeneticmaterial
throughthebacterialmembrane.Theinjectionisaccomplished throughasortof
bendingmotionintheshaftbygoingtotheside,contractingclosertothecelland
pushingbackup.

19. https://en.wikipedia.org/wiki/Escherichia_virus_T4
https://virologyj.biomedcentral.com/articles/10.1186/1743-422X-7-356
https://virologyj.biomedcentral.com/articles/10.1186/1743-422X-7-355