Bacteriophage 80α is a representative of a class of temperate phages that infect Staphylococcus aureus and other Gram-positive bacteria. Many of these phages carry genes encoding toxins and other virulence factors. This phage, 80α, is also involved in high-frequency mobilization of S. aureus pathogenicity islands (SaPIs), mobile genetic elements that carry virulence factor genes. Bacteriophage 80α encodes a minor capsid protein, gp44, between the genes for the portal protein and major capsid protein. Gp44 is essential for a productive infection by 80α but not for transduction of SaPIs or plasmids. We previously demonstrated that gp44 is an ejection protein that acts to promote progression to the lytic cycle upon infection and suggested that the protein might act as an anti-repressor of CI in the lytic-lysogenic switch. However, an 80α Δ44 mutant also exhibited a reduced rate of lysogeny. Here, we show that gp44 is a non-specific DNA binding protein with affinity for the blunt ends of linear DNA. Our data suggest a model in which gp44 promotes circularization of the genome after injection into the host cell, a key initial step both for lytic growth and for the establishment of lysogeny.
Keywords: exonuclease; genome replication; lytic–lysogenic switch; minor capsid protein; pilot protein; recombinase.