Characterization of Borrelia burgdorferi BlyA and BlyB proteins: a prophage-encoded holin-like system

The conserved cp32 plasmid family of Borrelia burgdorferi was recently show n to be packaged into a bacteriophage particle (C. H. Eggers and D. S. Samu els, J. Bacteriol. 181:7308-7313, 1999), This plasmid encodes BlyA, a 7.4-k Da membrane-interactive protein, and BlyB, an accessory protein, which were previously proposed to comprise a hemolysis system. Our genetic and bioche mical evidence suggests that this hypothesis is incorrect and that BlyA and BlyB function instead as a prophage-encoded holin or holin-like system for this newly described bacteriophage, An Escherichia coli mutant containing the blyAB locus that was defective for the normally cryptic host hemolysin SheA was found to be nonhemolytic, suggesting that induction of sheA by bly AB expression was responsible for the hemolytic activity observed previousl y, Analysis of the structural features of BlyA indicated greater structural similarity to bacteriophage-encoded holins than to hemolysins, Consistent with holin characteristics, subcellular localization studies with E. coli a nd B. burgdorferi indicated that BlyA is solely membrane associated and tha t BlyB is a soluble protein. Furthermore, BlyA exhibited a holin-like funct ion by promoting the endolysin-dependent lysis of an induced lambda lysogen that was defective in the holin gene. Finally, induction of the cp32 proph age in B. burgdorferi dramatically stimulated blyAB expression. Our results provide the first evidence of a prophage-encoded holin within Borrelia.