Moselio Schaechter

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December 19, 2013

Talmudic Question #104

Can you conceive of an entity that can replicate as a cell under some circumstances or as a virus under others?


Yes - it would have to be a kind of cross between a giant virus (Mimi- or Pandora- like) and a mycoplasma. In some conditions, it could infect host cells as a virus, with complete dissociation of the infecting particle at some point after entry, then DNA replication and formation of new infectious particles. In other conditions, the infectious particle itself could become capable of growth and division. In this case, the infectious particle would have to be something like a bacterial endospore - metabolically inert itself, but capable of producing a vegetative cell under the right conditions. Of course, you would need a large genome to code for these two different life cycles, but maybe 2Mb would be enough.

While such a beast may be a theoretical possibility, I am not sure it could exist in vivo. Viral lifestyles select for rapid replication, and loss of useless genes, so a few cycles of virus-style replication might select for trimmed down genomes that had lost essential genes for the cellular replication phase. Maybe the only way this could work in nature is if the two replication cycles had to alternate. But that would also require a rather unique set of environmental conditions. (Host cells periodically becoming very rare, thus favoring a cellular lifestyle, then nutrients becoming very sparse, thus favoring viral infection of host cells). The host would have to be some kind of autotroph, I guess, and once its population had been eradicated by the viral recplication cycle, the cellular form of our hypothetical organism could then grow as a heterotroph, feeding ghoulishly on the remains of the dead autotrophic host cells until everything is used up. This kind of thing could perhaps occur in a rather isolated environment, that is only rarely re-seeded by susceptible host autotrophs.

OK, now back to grading undergraduate lab reports.....


they can either spread by having kids or they could spread their ideas in a populution of humans.

A temperate phage replicates like a cell in lysogeny and like a virus when induced to its lytic cycle,

Does lysogeny count?

Oh yes, I can! One Deltaproteobacterium comes pretty close to the required specifications: Bdellovibrio bacteriovorus: it attaches to and invades Gram-negative bacteria (E. coli, Pseudomonas), resides in the periplasm of its host, sucks nutrients, grows & replicates and finally lyses the host to set its progeny free. This is a virus-like lifestyle. However, Bdello apparently doesn't engage any host factors for basic features as e.g. transcription, translation and replication, as most viruses do. And: it does not enter the cytoplasm of its host. In the lab, you can get Bdello mutants that grow prey-independently – even axenically – thus they 'replicate as cells'.
P.S. I assume that Mycoplasma laboratorium does not count as answer to question #104 since it was created artificially in the Craig Venter lab.

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