The A ^* protein of φX174 is an inhibitor of DNA replication

Extracts prepared from φX174 infected E.coli cells inhibited in vitro RF replication The inhibition was dependent upon the presence of A ^* protein in the reaction and served as an assay to highly purify the A ^* protein. Purified A ^* protein bound tightly to duplex DNA as well as single-stranded DNA. The binding of the A ^* protein to duplex DNA inhibited (I) its single-stranded DNA specific endonucleolytic activity; (II) invitro synthesis of viral (+) single stranded DNA on an A-RFII DNA complex template; (III) ATP hydrolysis by rep protein and unwinding of the strands of RF DNA.

We propose that this inhibitory activity is responsible invivo for the shut off of E.coli chromosome replication during φX174 infection, and has a role in the transition from semiconservative RF DNA replication to single-stranded DNA synthesis in the life cycle of φX174.