RESISTANCE STRATEGIES DIFFER AGAINST GENERALIST AND SPECIALIST CYANOPHAGES

Cyanobacteria of the genera Synechococcus and Prochloroccocus and the viruses that infect them are abundant in the marine environment. Coexistence between them is thought to be facilitated by the presence of both resistant and susceptible cells. While extracellular resistance by alterations in receptors is well recognized, known intracellular defense mechanisms are lacking in these cyanobacteria. In order to investigate whether intracellular resistance exists in Prochlorococcus and Synechococcus we sought to identify infections that are halted inside the cell. We found that generalist T4-like cyanophages tend to adsorb to resistant hosts more often than specialist phages suggesting that resistance to specialist phages is generally extracellular. In order to assess whether phages that adsorb to resistant hosts enter the cell, we next tested for DNA expression and replication of these phages inside resistant strains. This revealed that the DNA of both generalists and specialists phages entered the cell and was expressed. In addition, we often observed replication of the viral genome in these infections. In one such infection, we tested for protein synthesis and found that while translation occurred in the resistant strain, multiple morphogenesis and DNA packaging proteins were lacking. These data indicate that resistance to generalist phages is often conferred inside the cell and may be due to novel intracellular mechanisms of defense.