Transcriptional regulation and immunity in mycobacteriophage Bxb1

Mycobacteriophage Bxb1 is a temperate phage of Mycobacterium smegmatis that shares a similar genome organization to mycobacteriophage L5, although the two phages are heteroimmune. We have investigated the regulatory circuitry of Bxb1 and found that it encodes a repressor, gp69, which regulates at le ast two promoters, an early lytic promoter, P-left, and the divergent promo ter, P-right. Bxb1 gp69 is 41% identical to the L5 repressor (gp71) and bin ds to repressor binding sites that conform to a similar, but distinct, 13 b p asymmetric consensus sequence to that for the L5 gp71 binding sites. The two phage repressors have a strong preference for their cognate binding sit es, thus accounting for their immunity phenotypes. The Bxb1 genome contains 34 putative repressor binding sites located throughout the genome, but sit uated within short intergenic spaces and orientated in only one direction r elative to the direction of transcription. Comparison with the locations of repressor binding sites within the L5 genome provides insights into how th ese unusual regulatory systems evolve.