LytF contributes to pilus extrusion during natural competence in Streptococcus sanguinis SK36

Streptococci may enter a physiological state called competence, during which they express a specific set of genes required for exogenous DNA uptake and its subsequent integration into the genome through homologous recombination. This process, termed natural transformation, facilitates the horizontal acquisition of genetic material, potentially conferring adaptive advantages that enhance bacterial survival under selective pressures. To make homologous DNA available in the surrounding environment, Streptococcus pneumoniae expresses a cell wall hydrolase (CbpD) that lyses and kills closely related species. This process has been coined fratricide, and the acting hydrolase a fratricin. A significant fraction of streptococcal species does not encode a CbpD-like protein, but another competence-induced peptidoglycan hydrolase LytF. It has been speculated that LytF serves the same purpose as CbpD; however, our investigations into the role of LytF in Streptococcus sanguinis revealed no evidence supporting LytF as a fratricin. Instead, we show that LytF is involved in natural transformation by promoting DNA uptake. An essential part of DNA uptake is the competence-induced type IV pilus, which facilitates DNA uptake by pulling nearby DNA toward the cell. By immunoblotting and microscopy imaging, we found that LytF increases extracellular levels of the major pilus component ComGC, suggesting that LytF may modify peptidoglycan to promote pilus extrusion across the cell wall, thereby enhancing the efficiency of DNA uptake.IMPORTANCEStreptococci are a significant cause of severe infections in both humans and animals. They are particularly adept at acquiring new genes through horizontal gene transfer, as they can become competent for natural transformation. This allows them to quickly adapt to selective pressure and spread genes involved in virulence and antibiotic resistance. In Streptococcus sanguinis, the competence-induced peptidoglycan hydrolase LytF has been reported to stimulate natural transformation. Our study contributes to understanding this process by demonstrating that LytF promotes extrusion of the transformation pilus required for DNA uptake.