Competence-Dependent Bacteriocin Production in Streptococcus gordonii DL1 (Challis)

Bacteriocins (antimicrobial peptides secreted by bacteria) are assumed to function in anticompetitor strategies. The characteristics of many bacteriocins have been elucidated but their ecological functions remain largely unstudied and thus unresolved. Objective: The aim of this study was to establish whether a genetic relationship exists between natural competence for genetic transformation and the production of bacteriocins STH1 and STH2 in Streptococcus gordonii strain DL1. Methods: Several genes known to be centrally involved in control of competence development in strain DL1 were inactivated by insertion of an erythromycin resistance element (ermAM). Resulting non-competent mutants were tested for bacteriocin expression using several indicator organisms. A phenotypic complementation strategy was used to identify bacteriocin putative structural genes. Results: Individual inactivation of selected competence regulatory genes (comC, comAB, comE and comR) resulted in mutants unable to express bacteriocin activity, indicating that bacteriocin expression is indeed competence-dependent. Inactivation of comA, which codes for the ABC transporter responsible for exporting the competence stimulating peptide (CSP), prevented bacteriocin expression even by CSP-stimulated bacteria, suggesting that the bacteriocins are also secreted by this transporter. The putative bacteriocin structural genes, sthA and sthB, were identified within the same regulon and their functions confirmed by insertional inactivation and episomal complementation. Both genes encode short (< 60-residue) hydrophobic prepeptides possessing potential double-glycine leader cleavage sites, suggesting secretion by ABC transporters. A recognition site (cin-box) for the competence sigma factor ComR was identified directly upstream of sthA. Conclusion: These findings indicate that bacteriocin expression is controlled by competence regulatory genes in strain DL1, suggesting that bacteriocins STH1 and STH2 enhance transformation efficacy. This could be achieved either by excluding strains competing for available DNA or by killing related bacteria to release potentially useful genetic material. The authors gratefully acknowledge the support of the Marsden Fund of New Zealand (contract no. 99-UOO-099).