Cyanobacteria of the genera Synechococcus and Prochloroccocus are widespread and contribute significantly to oceanic primary production. Viruses that infect cyanobacteria are also abundant in the marine environment and affect cyanobacterial populations by causing host mortality during lytic infection. For a virus to successfully reproduce inside a bacterial cell it must first recognize and adsorb to the cell, enter it, express and replicate its genome, build new virions and lyse the cell to exit. In addition to actively infected cyanobacterial strains, we found numerous cyanobacterial strains which were not lysed when infected with the Syn9 virus and can thus be considered resistant to infection by this virus. However, Syn9 adsorbs to six such resistant cyanobacterial strains. This suggests that resistance is achieved at a stage downstream of recognition and attachment. In order to determine whether the viral DNA enters the cell after adsorption we tested for viral gene expression and DNA replication using quantitative reverse transcriptase PCR and quantitative PCR, respectively. This revealed that viral RNA was expressed in all the resistant strains. The extent of progression of gene expression, however, varied depending on the infected strain. In addition, we found a resistant Synechococcus strain in which the virus genome was also replicated. However, no viral progeny were produced, nor was there assembly of infective viruses inside the cell. These data indicate that resistance is conferred inside the cell and may be due to novel intracellular mechanisms of defense. Identifying these mechanisms is the topic of further investigation.
