The complete genomic sequence of the marine phage Roseophage SIO1 shares homology with nonmarine phages

Viruses are ubiquitous components of the marine environment, frequently rea ching concentrations of 10(7)-10(8) viruses per milliliter of surface seawa ter The majority of these viral particles are bacteriophages (phages). Alth ough the oceans are probably the largest pool of bacteriophages on the plan et, the evolutionary relationships of marine phages to phages from other en vironments are unknown. To address this issue, we have completely sequenced the genome of the lytic marine phage, Roseophage SIO1, that infects the he terotrophic marine bacterium Roseobacter SIO67. This phage has an isometric capsid with a diameter of approximately 43 nm, a short tail, a buoyant den sity of 1.49 g cm(-3) in CsCl, and a 39,906-bp dsDNA genome. Sequence simil arities and relative positions within the genome suggest that three of the open reading frames (ORFs) are homologous to the primase, DNA polymerase, a nd endodeoxyribonuclease I proteins of coliphages T3 and T7. The results ar e consistent with the mosaic theory of phage evolution and indicate a genet ic link between marine and nonmarine phages. Additionally, basic life histo ries of marine phages can be elucidated by comparison of complete genomes t o those of other extensively studied phages (e.g., lambda, T4, T7). The DNA replication machinery of Roseophage SIO1 shows a clear homology with that of coliphages T3 and T7, suggesting that the process of DNA replication may be similar among these phages. The Roseophage SIO1 genome also encodes fou r predicted proteins involved in phosphate metabolism (RP PhoH, RP ribonucl eotide reductase, RP Thy1, and RP endodeoxyribonuclease I) suggesting that phosphate recycling is important to Roseophage SIO1's life cycle. Other int eresting clues about Roseophage SIO1's life history come from the absence o f certain expected protein regions. For example, we have not been able to i dentify the Roseophage SIO1 structural proteins (e.g., capsid proteins) by homology to other phages. It is also conspicuous that the Roseophage SIO1 g enome lacks a recognizable RNA polymerase, an essential component of T3 and T7 life cycles. Analysis of the Roseophage SIO1 genome shows that marine a nd nonmarine phages are genetically related but basic life histories may be significantly different.