Fratricide - streptococci getting access to homologous DNA during the competent state

Orateur : Leiv Sigve HAVARSTEIN (Norwegian University of Life Sciences Department of Chemistry, Biotechnology and Food Science)

Bacteria that are competent for natural transformation have the ability to take up and integrate free DNA from their environment. Most competent bacteria live in multispecies biofilms containing foreign DNA from unrelated bacteria whose uptake and integration might be harmful. A number of naturally transformable bacteria live in multispecies biofilms in the oral cavity and nasopharynx. To avoid foreign DNA, naturally transformable members of the Neisseriaceae and Pasteurellaceae families preferentially take up DNA with conserved sequence motifs (USS or DUS, about 10 bp long). Streptococcus pneumoniae are USS/DUS-free and will take up DNA from any source during the competent state which allows access to homologous donor DNA through fratricid : the killing of species S. pneumoniae and closely related species. During competence, in liquid cultures and biofilms, pneumococci produce a murein hydrolase, CbpD, which kills and lyses non-competent pneumococci. It has been proposed that CbpD constitutes a DNA-acquisition mechanism enabling competent pneumococci to capture homologous DNA from susceptible streptococci sharing the same habitat. The exact mechanism by which the integral membrane protein ComM protects competent pneumococci against CbpD is not known. The protein is toxic to the cell if overproduced, and the level of ComM is closely controlled by rapid intramembrane proteolysis. The genomes of a number of streptococci do not encode CbpD-type proteins. Our studies show that the genomes of nearly all species lacking CbpD encode LytF, an unrelated competence-regulated murein hydrolase. We obtained evidence indicating that LytF is a functional analogue of CbpD. Our results indicate that a murein hydrolase gene is part of the competence regulon of all streptococcal species, demonstrating that these fratricins constitute an essential part of the streptococcal natural transformation system.

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