Putative prophages related to lytic tailless marine dsDNA phage PM2 are widespread in the genomes of aquatic bacteria

Here we identified thirteen putative prophages (viral genomes integrated into bacterial chromosome) closely related to the virulent icosahedral tailless lipid-containing bacteriophage PM2. Using the comparative genomics approach, we present evidence to support the viral "self" hypothesis and divide genes of the bacteriophage PM2 and related prophages into "self" and "nonself" categories.We show here that the previously proposed most conserved viral "self" determinants, the major coat protein and the packaging ATPase, were the only proteins that could be recognized in all detected corticoviral elements. We also argue here that the genes needed for viral genome replication, as well as for host cell lysis, belong to the "nonself" category of genes.Furthermore, we suggest that abundance of PM2-like viruses in the aquatic environment as well as their importance in the ecology of aquatic microorganisms might have been underestimated.Recently observed structural relationships between the architectures of viruses infecting hosts from the three domains of life [1-4] poured fuel onto the discussion of the origin and evolution of viruses [5-11]. It is now evident that viruses are ancient and that very diverse virus families may have arisen from a relatively small number of primordial viral ancestors. We have suggested previously that a limited number of features represent the essence of a virus and that viruses sharing such characteristics can be joined into lineages. Those features are responsible for the formation of a functional virion and are referred here as the viral "self" (discussed in [5,6,8,12,13]). On the other hand, components encoded in viral genome and needed for interaction with the host are assumed to be nonessential for the virion formation and thus represent the "nonself" category, although they are indispensable for viral genome replication and the interaction of the virus with its host.Ascribing viral properties to "self" or "nonself" categories for highly