Release of latent ClyA cytolysin from Escherichia coli mediated by a bacteriophage-associated putative holin (BlyA) from Borrelia burgdorferi.

Introduction of the Borrelia burgdorferi blyAB locus into Escherichia coli has recently been reported to cause a hemolytic phenotype that is dependent on the E. coli clyA (hlyE, sheA) gene (a cytolysin gene present in many E. coli strains, including E. coli K-12, which is repressed under standard in vitro growth conditions). The blyA gene product has been suggested to be a prophage-encoded holin, but the processes triggered in E. coli by the expression of blyA and/or blyB, which lead to the hemolytic phenotype, remained unclear. Here we show that expression of blyA in E. coli causes damage to the E. coli cell envelope and a clyA-dependent hemolytic phenotype, regardless whether blyB is present or absent. The expression of blyB in E. coli, on the other hand, did not have obvious phenotypic effects. Transcriptional studies demonstrated that the clyA gene is not induced in E. coli cells expressing blyA. Furthermore, protein analyses suggested that the impairment of the E. coli cell envelope by BlyA is responsible for the emergence of the hemolytic activity as it allows latent intracellular ClyA protein, derived from basal-level expression of the clyA gene, to leak into the medium and to lyse erythrocytes. These findings are compatible with the presumption that BlyA functions as a membrane-active holin.