THE COM LOCUS CONTROLS GENETIC-TRANSFORMATION IN STREPTOCOCCUS-PNEUMONIAE

Genetic exchange by natural transformation in Streptococcus pneumoniae occurs in a cell-density dependent process and is initiated by a smal l extracellular signalling molecule, the competence-stimulating peptid e (CSP). comC, the gene for this peptide, has previously been identifi ed and encodes a 44 amino acid pre-peptide that is apparently processe d to an active molecule that consists of the C-terminal 17 amino acids . We have sequenced the region adjacent to comC and shown that it is t he first gene of an operon, com, consisting of two downstream elements , comD and comE, which encode members of the two-component family of s ensor regulators. Null mutants with defects in either comC or comD wer e transformation deficient and failed to respond to exogenous CSP. A c omC mutant did not exhibit any detectable CSP activity, white a comD m utant that contained an intact comC produced minimal CSP activity. In mixed-culture experiments consisting of isogenic pairs of pneumococci (Csp(+) and Csp(-)), we showed that induction of competence by quorum sensing was independent of CSP. Northern analysis showed that corn was transcribed as a single polycistronic message, while analysis of stra ins with transcriptional fusions showed that corn was constitutively e xpressed under conditions that both promoted or repressed the developm ent of competence. Finally, we showed genetically and biochemically a CSP-dependent transcription of rec, a competence-induced locus, and th at ComD and ComE are required for this CSP-dependent expression.