HALTED VIRAL INFECTIONS OF MARINE CYANOBACTERIA HINT AT NOVEL INTRACELLULAR HOST DEFENSE MECHANISMS

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. Resistance to viruses is largely considered one of the prominent mechanisms that prevent viral populations from eliminating their hosts. While evidence exists for extracellular resistance by lack of adsorption in both Prochlorococcus and Synechococcus, no evidence has been found for any of the known intracellular defense mechanisms in these cyanobacteria. In order to investigate whether intracellular defense mechanisms exist in marine Prochlorococcus and Synechococcus we sought to identify infections halted at intracellular stages of the infection process. We found that broad host range phages tend to adsorb to resistant hosts more often than narrow host range phages. This indicates that resistance to broad host range phages is achieved at a stage downstream of recognition and attachment. To determine whether viral DNA enters the cell after adsorption we tested for viral gene expression and DNA replication of a broad host range phage. This revealed that viral RNA was expressed in all the resistant strains the phage adsorbed to. 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 underwent replication. However, no viral progeny were produced, nor was there encapsidation of viral DNA inside the cell. These data indicate that resistance is conferred inside the cell in many cyanobacterial strains and may be due to novel intracellular mechanisms of defense. Identifying these mechanisms is the topic of further investigation.