# Quorum Sensing Regulation of Competence and Bacteriocins in _Streptococcus pneumoniae_ and _mutans_
## 4 Bacteri Regulation and Function in _S. pneumoniae_  
Entry into the state of competence is an energetically costly endeavor, resulting in the coordinated expression of over 100 CSP-responsive genes [20]. For _S. pneumoniae_ to benefit from a state of competence, a source of \"naked\", or extracellular DNA must also be available. This DNA could be derived from closely related strains, more distantly related species, or even from virtually identical sibling cells. Therefore, it is not surprising that _S. pneumoniae_ has evolved mechanisms to ensure access to DNA. One such mechanism, termed allolysis or fatricide, results from the targeted lysis of non-competent _S. pneumoniae_ cells by their competent siblings, hence the term fatricide [42; 43].  
At least six genes implicated in fatricide are up regulated in _S. pneumoniae_ in response to CSP stimulation. These include the previously mentioned early gene _comM_, as well as _cibABC_, _cbpD_, and _lytA_, all of which are induced by ComX alongside other late competence genes [20]. CibAB consists of a two-peptide bacteriocin responsible for lysis of cells lacking the corresponding immunity factor, CibC [44]. Bacteriocins are a class of small, ribosomally-synthesized peptides that exhibit antimicrobial activity towards bacteria that lack the corresponding immunity factor [45]. Therefore, a competent _S. pneumoniae_ cell would express CibABC and would then be capable of targeting a nearby non-competent _S. pneumoniae_ cell that is unable to express functional CibC. Generally, type II bacteriocins, including CibAB, are thought to exert cidal effects on target cells through mechanisms ultimately leading to membrane leakage. For example, type IIa bacteriocins, representing a subclass of pediccin-like linear un-modified peptides, bind to the mannose-phosphor transferase system (Man-PTS) on a target cell, initiating the formation of a pore [46] (recently reviewed by [PERSON] [47]). CbpD is a murein hydrolase containing a cytosine, histidine-dependent amidohydrolase peptidase (CHAP) domain that functions extracellularly to degrade murein stem peptides found in the pneumococcal cell wall, resulting in structural changes that promote lysis [44]. Competence-induced expression of the early gene _comM_, encoding a membrane-bound protein, confers immunity to CbpD-mediated lysis [39]. For fatricide to occur in liquid culture, full CbpD activity requires LytA, an effector of autolysis in _S. pneumoniae_[42; 43; 48]. Furthermore, a separate lytic amidase, not known to be regulated by competence, LytC, is also required for complete competence-induced fatricide [44].  
Interestingly, both of these mechanisms of predation appear to be restricted to isogenic or closely related strains [49; 50]. This observation fits with the suggestion that competent cells target related cells to acquire homologous DNA sequences to either maintain genome integrity or to acquire new gene alleles from the pneumococcal meta-gene pool to provide phenotypic heterogeneity and diversity that could enhance fitness within the community. The likelihood for transformation correlates directly with increasing homology between the donor's DNA and the recipient's genome since success relieson homologous recombination [6]. However, _cibABC/cbpD/lyrA_-dependent transformation may not entirely account for the apparent extensive horizontal gene transfer present in pneumococcus.