Fratricide in Streptococcus pneumoniae: contributions and role of the cell wall hydrolases CbpD, LytA and LytC

Pneumococci that have developed the competent state kill and lyse non-competent sister cells and members of closely related species during co-cultivation in vitro. The key component in this process, called fratricide, is the product of the late competence gene cbpD. In addition, the peptidoglycan hydrolases LytA and LytC are required for efficient lysis of target cells. Here, we have investigated the relative contribution and possible role of each of the proteins mentioned above. Previous studies have shown that CbpD is produced exclusively by competent cells, whereas LytA and LytC can be provided by the competent attackers as well as the non-competent target cells. By using an improved assay to compare the effect of cis- versus trans-acting LytA and LytC, we were able to show that target cells are lysed much more efficiently when LytA and LytC are provided in cis, i.e. by the target cells themselves. Western analysis demonstrated that considerable amounts of LytC are present in the growth medium. In contrast, we were not able to detect any extracellular LytA. This finding indicates that LytA- and LytC-mediated fratricide represent different processes. In the absence of LytA and LytC, only a tiny fraction of the target cells were lysed, demonstrating that CbpD does not function efficiently on its own. However, in the presence of 1 mM EDTA, the fraction of target cells lysed directly by CbpD increased dramatically, indicating that divalent cations are involved in the regulation of fratricide under natural conditions.