Genome organization and characterization of mycobacteriophage Bxb1

Mycobacteriophage Bxb1 is a temperate phage of Mycobacterium smegmatis. The morphology of Bxb1 particles is similar to that of mycobacteriophages L5 a nd D29, although Bxb1 differs from these phages in other respects. First, i t is heteroimmune with L5 and efficiently forms plaques on an L5 lysogen. S econdly, it has a different host range and fails to infect slow-growing myc obacteria, using a receptor system that is apparently different from that o f L5 and D29. Thirdly, it is the first mycobacteriophage to be described th at forms a large prominent halo around plaques on a lawn of M. smegmatis. T he sequence of the Bxb1 genome shows that it possesses a similar overall or ganization to the genomes of L5 and D29 and shares weak but detectable DNA sequence similarity to these phages within the structural genes. However, B xb1 uses a different system of integration and excision, a repressor with d ifferent specificity to that of L5 and encodes a large number of novel gene products including several with enzymatic functions that could degrade or modify the mycobacterial cell wall.