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COMC family This family consists exclusively of streptococcal competence stimulating peptide precursors, which are generally up to 50 amino acid residues long. In all the members of this family, the leader sequence is cleaved after two conserved glycine residues; thus the leader sequence is of the double- glycine type. Competence stimulating peptides (CSP) are small (less than 25 amino acid residues) cationic peptides. The N-terminal amino acid residue is negatively charged, either glutamate or aspartate. The C-terminal end is positively charged. The third residue is also positively charged: a highly conserved arginine. A few COMC proteins and their precursors (not included in this family) do not fully follow the above description. In particular: the leader sequence in the CSP precursor from Streptococcus sanguis NCTC 7863 is not of the double-glycine type; the CSP from Streptococcus gordonii NCTC 3165 does not have a negatively charged N-terminus residue and has a lysine instead of arginine at the third position. Functionally, CSP act as pheromones, stimulating competence for genetic transformation in streptococci. In streptococci, the (CSP mediated) competence response requires exponential cell growth at a critical density, a relatively simple requirement when compared to the stationary-phase requirement of Haemophilus, or the late-logarithmic- phase of Bacillus. All bacteria induced to competence by a particular CSP are said to belong to the same pherotype, because each CSP is recognized by a specific receptor (the signalling domain of a histidine kinase ComD). Pherotypes are not necessarily species-specific. In addition, an organizm may change pherotype. There are two possible mechanisms for pherotype switching: horizontal gene transfer, and accumulation of point mutations. The biological significance of pherotypes and pherotype switching is not definitively determined. Pherotype switching occurs frequently enough in naturally competent streptococci to suggest that it may be an important contributor to genetic exchange between different bacterial species. The family Antibacterial16, streptolysins from group A streptococci, has been merged into this family. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",