Cyanobacteria from the genus Synechococcus are highly abundant in the oceans where they contribute significantly to primary production. Lytic viruses are thought to have a major impact on cyanobacterial population dynamics and evolution. Previously, during investigation of the transcriptional response of 3 Synechococcus strains to infection by the Syn9 cyanophage, we found that while the transcript levels of the vast majority of genes declined soon after infection, those for some genes increased or remained stable. In order to assess the role of two such genes during infection, we inactivated them in Synechococcus strain WH8102. One gene, SYNW1659, encodes a domain of unknown function and is associated with restriction enzymes. The second gene, SYNW1946, encodes a single-stranded RNA nuclease PIN domain and a PhoH domain which is common in phage genomes including Syn9. We found that neither mutation impacted host growth rate and that the length of the Syn9 infection cycle remained the same as in the wild type. However, the SYNW1659 mutant allowed for a significantly lower yield of phage progeny suggesting that the product of this gene may aid phage production. The SYNW1946 mutant, on the other hand, allowed for two fold higher Syn9 genomic DNA replication suggesting that this gene may play a role in restraining the infection process. Our findings suggest that some host genes that respond to phage infection are likely to be recruited by the phage for improved infection while others may function in an attempt at defense against the phage.
