News from the De­part­ment of Mimivi­rol­ogy: Part II

by Elio

New and tan­ta­liz­ing facts con­tinue to emerge from those on the front­lines study­ing the gi­ant viruses. A re­cent pa­per by La Scola et al. re­ports the dis­cov­ery of a novel virus found in amoe­bas that re­quires mimivirus for repli­ca­tion. This de­fec­tive virus has been called Sput­nik (for fel­low trav­eler). This ex­cit­ing find­ing ex­pands even fur­ther the fron­tiers of vi­rol­ogy.

Sput­nik in­side ma­mavirus cap­sid. Source

Such news elic­its spir­ited de­bate and thought­ful com­men­tary in the bl­o­gos­phere, such as here and here. We asked two vi­rol­o­gists for com­ment, one from the per­spective of the adeno-as­so­ci­ated viruses, the other from the world of phages.

by Ken Berns

La Scola et al. con­sider Sput­nik a novel phe­nom­e­non and wish to term it a vi­rophage be­cause, in or­der to repli­cate, it re­quires co-in­fec­tion with the virus APMV4 and be­cause its own repli­ca­tion in­hibits that of its helper. In­deed, this is iden­ti­cal to the be­hav­ior of adeno-as­so­ci­ated viruses (AAV). They, too, re­quire co-in­fec­tion with a helper virus – ei­ther an adeno- or a her­pesvirus – for pro­duc­tive in­fec­tion. If the co-in­fec­tion is si­mul­ta­ne­ous, ade­n­ovirus repli­ca­tion can be se­verely in­hib­ited. Also, sev­eral her­pes helpers are in­hib­ited by AAV repli­ca­tion. Thus, Sput­nik is not a novel bi­o­log­i­cal phe­nom­e­non, but a nice ex­am­ple of an ar­chaeal virus whose be­hav­ior is sim­i­lar to a well-char­ac­ter­ized hu­man virus.

Ken Berns di­rects the Ge­net­ics In­sti­tute at the Uni­ver­sity of Florida. He is past pres­i­dent of the ASM and an au­thor­ity on adeno-as­so­ci­ated viruses. 

by Richard Cal­en­dar

Satel­lite (or helper-de­pen­dent) col­iphage P4 de­pends upon helper phage P2 cap­sid, tail and ly­sis pro­teins. P4 makes a phage par­ti­cle with a P2 tail and a smaller, mod­i­fied P2 cap­sid, ap­pro­pri­ate for the size of P4 DNA. P4 in­hibits the growth of its P2 helper. Thus, al­though their state­ment* is in­cor­rect, the find­ing of the Sput­nik phage is cer­tainly in­ter­est­ing.

*From the pa­per by La Scola et al.: To our knowl­edge, this ob­ser­va­tion of a virus us­ing the vi­ral fac­tory of an­other virus to prop­a­gate at the ex­pense of its vi­ral host has not been de­scribed pre­vi­ously.

Richard Cal­en­dar is Pro­fes­sor of Bio­chem­istry and Mol­e­c­u­lar Bi­ol­ogy at the Uni­ver­sity of Cal­i­for­nia at Berke­ley and the ed­i­tor of the widely ac­claimed book, The Bac­te­rio­phages. For his re­view cov­er­ing the strate­gies of satel­lite phage P4, click here.