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Competence for genetic transformation in  Streptococcus pneumoniae  is regulated by a quorum-sensing system encoded by two genetic loci,  comCDE  and  comAB . Additional competence-specific operons,  cilA ,  cilB , cilC ,  cilD ,  cilE , cinA-recA ,  coiA , and  cfl , involved in the DNA uptake process and recombination, share an unusual consensus sequence at −10 and −25 in the promoter, which is absent from the promoters of  comAB  and  comCDE . This pattern suggests that a factor regulating transcription of these transformation machinery genes but not involved with  comCDE  and comAB  expression might be an alternative sigma factor. A search for such a global transcriptional regulator was begun by purifying pneumococcal RNA polymerase holoenzyme. In preparations from competent pneumococcal cultures a protein which seemed to be responsible for  cilA  transcription in vitro was identified. The corresponding gene was identified and found to be present in two copies, designated  comX1  and  comX2 , located adjacent to two of the repeated rRNA operons. Expression of transformation machinery operons, such as  cilA , cilD ,  cilE , and  cfl , but not that of the quorum-sensing operons  comAB  and  comCDE , was shown to depend on  comX , while  comX expression depended on ComE but not on ComX itself. We conclude that the factor is a competence-specific global transcription modulator which links quorum-sensing information transduced to ComE to competence and propose that it acts as an alternate sigma factor. We also report that  comAB  and  comCDE  are not sufficient for shutoff of competence-stimulating peptide-induced gene expression nor for the subsequent refractory period, suggesting that these phenomena depend on one or more ComX-dependent genes.

Identification of a New Regulator inStreptococcus pneumoniae Linking Quorum Sensing to Competence for Genetic Transformation