An Unusual Phage Repressor Encoded by Mycobacteriophage BPs

Temperate bacteriophages express transcription repressors that maintain lysogeny by down-regulating lytic promoters and confer superinfection immunity. Repressor regulation is critical to the outcome of infection—lysogenic or lytic growth—as well as prophage induction into lytic replication. Mycobacteriophage BPs and its relatives use an unusual integration-dependent immunity system in which the phage attachment site (attP) is located within the repressor gene (33) such that site-specific integration leads to synthesis of a prophage-encoded product (gp33103) that is 33 residues shorter at its C-terminus than the virally-encoded protein (gp33136). However, the shorter form of the repressor (gp33103) is stable and active in repression of the early lytic promoter PR, whereas the longer virally-encoded form (gp33136) is inactive due to targeted degradation via a C-terminal ssrA-like tag. We show here that both forms of the repressor bind similarly to the 33–34 intergenic regulatory region, and that BPs gp33103 is a tetramer in solution. The BPs gp33103 repressor binds to five regulatory regions spanning the BPs genome, and regulates four promoters including the early lytic promoter, PR. BPs gp33103 has a complex pattern of DNA recognition in which a full operator binding site contains two half sites separated by a variable spacer, and BPs gp33103 induces a DNA bend at the full operator site but not a half site. The operator site structure is unusual in that one half site corresponds to a 12 bp palindrome identified previously, but the other half site is a highly variable variant of the palindrome.