Identification and characterization of the loci encoding the competence-associated alternative sigma factor of Streptococcus gordonii

In naturally-competent streptococci such as Streptococcus pneumoniae, expression of the late competence operons is regulated by ComX (sigma(X)), the competence-specific alternative sigma factor. In this study, duplicate genes (comR1 and comR2) encoding the putative ComX homologue of the oral bacterium Streptococcus gordonii were identified. Like the identical twin comX loci of S. pneumoniae, both comR determinants are independently functional as well as responsive to the ComDE signal transduction system activated by competence-stimulating peptide. However, in contrast to the comX system, nucleotide sequence analyses in combination with in trans complementation studies with a comR null mutant demonstrate that the identical 83 bp tracts (Region I) located immediately upstream of the comR structural genes are insufficient to confer wild-type competence levels. Wild-type transformation levels required additional distal nonhomologous DNA segments (Region II). Our findings suggest that alternative regulatory elements, under overall control of the ComDE pathway, may influence expression of the comR loci.     

Identification of ComW as a new component in the regulation of genetic transformation in Streptococcus pneumoniae

Regulation of competence for genetic transformation in Streptococcus pneumoniae depends on a quorum-sensing system, genes involved in DNA uptake and recombination and a link between these two gene sets. The alternative sigma factor ComX provides this link. ComE, the response regulator of the quorum-sensing system, is required for expression of ComX and other early genes. However, an unknown ComE-dependent regulator is also required for competence when comX is expressed under control of the raffinose-responsive promoter of the aga operon. The gene comW (SP0018) is required for a high level of competence and is regulated by the quorum-sensing system, but its function is unknown. To explore its role further, comW was cloned into the multicopy plasmid pMSP3535, under the control of a nisin-inducible promoter (P(N)), and transformed into pneumococcal strains containing a raffinose-inducible comX gene (P(R)::comX). Further introduction of a comE deletion blocked the endogenous CSP signal transduction pathway. In the resulting strain, competence was independent of CSP but depended on treatment with both nisin and raffinose, showing that coexpression of comW and comX complemented the comE deficiency. ComX protein accumulation and expression of a late competence gene in the above strain support the conclusion that ComW is a new positive factor involved in competence regulation.     

Conservation of key elements of natural competence in Lactococcus lactis ssp

Natural competence is active in very diverse species of the bacterial kingdom and probably participates in horizontal gene transfer. Recently, the genome sequence of various species, including Lactococcus lactis, revealed the presence of homologues of competence genes in bacteria, which were not previously identified as naturally transformable. We investigated the conservation among lactococcal strains of key components of the natural competence process in streptococci: (i) comX which encodes a sigma factor, allowing the expression of the late competence genes involved in DNA uptake, (ii) its recognition site, the cin-box and (iii) dprA which encodes a protein shown to determine the fate of incoming DNA. The comX and dprA genes and the cin-box appeared conserved among strains, although some L. lactis ssp. lactis strains presented an inactivated dprA gene. We established that ComX controls the expression of the late competence genes in L. lactis. In conclusion, our work strongly suggests that ComX has the same role in streptococci and L. lactis, i.e. the regulation of late competence genes. It also allowed the identification of a set of L. lactis strains and the construction of a comX overexpression system, which should facilitate the investigation of the natural competence activity in lactococci.     

A MecA paralog, YpbH, binds ClpC, affecting both competence and sporulation

ComK, the master regulator of competence, is degraded by the general stress-related protease ClpCP but must be targeted to this protease by binding to the adapter protein MecA. The genome of Bacillus subtilis contains a paralog of mecA, ypbH. We show in the present study that YpbH, like MecA, binds ClpC and that its elimination or overproduction affects competence and sporulation.     

The Streptococcus sanguinis competence regulon is not required for infective endocarditis virulence in a rabbit model

Streptococcus sanguinis is an important component of dental plaque and a leading cause of infective endocarditis. Genetic competence in S. sanguinis requires a quorum sensing system encoded by the early comCDE genes, as well as late genes controlled by the alternative sigma factor, ComX. Previous studies of Streptococcus pneumoniae and Streptococcus mutans have identified functions for the >100-gene com regulon in addition to DNA uptake, including virulence. We investigated this possibility in S. sanguinis. Strains deleted for the comCDE or comX master regulatory genes were created. Using a rabbit endocarditis model in conjunction with a variety of virulence assays, we determined that both mutants possessed infectivity equivalent to that of a virulent control strain, and that measures of disease were similar in rabbits infected with each strain. These results suggest that the com regulon is not required for S. sanguinis infective endocarditis virulence in this model. We propose that the different roles of the S. sanguinis, S. pneumoniae, and S. mutans com regulons in virulence can be understood in relation to the pathogenic mechanisms employed by each species.