Development of Competence for Genetic Transformation of Streptococcus mutans in a Chemically Defined Medium

Streptococcus mutans develops competence for genetic transformation in response to regulatory circuits that sense at least two peptide pheromones. One peptide, known as CSP, is sensed by a two-component signal transduction system through a membrane receptor, ComD. The other, derived from the primary translation product ComS, is thought to be sensed by an intracellular receptor, ComR, after uptake by oligopeptide permease. To allow study of this process in a medium that does not itself contain peptides, development of competence was examined in the chemically defined medium (CDM) described by van de Rijn and Kessler (Infect. Immun. 27:444, 1980). We confirmed a previous report that in this medium comS mutants of strain UA159 respond to a synthetic peptide comprising the seven C-terminal residues of ComS (ComS(11-17)) by increasing expression of the alternative sigma factor SigX, which in turn allows expression of competence effector genes. This response provided the basis for a bioassay for the ComS pheromone in the 100 to 1,000 nM range. It was further observed that comS(+) (but not comS mutant) cultures developed a high level of competence in the late log and transition phases of growth in this CDM without the introduction of any synthetic stimulatory peptide. This endogenous competence development was accompanied by extracellular release of one or more signals that complemented a comS mutation at levels equivalent to 1 mu M synthetic ComS(11-17).