Competence-specific induction of recA is required for full recombination proficiency during transformation in Streptococcus pneumoniae

Abstract

Transcriptional activation of the recA gene of Streptococcus pneumoniae was previously shown to occur at competence. A 5.7 kb recA-specific transcript that contained at least two additional genes, cinA and dinF, was identified. We now report the complete characterization of the recA operon and investigation of the role of the competence-specific induction of recA. The 5.7 kb competence-specific recA transcript is shown to include lytA, which encodes the pneumococcal autolysin, a protein previously shown to contribute to virulence of S. pneumoniae. Uncoupling (denoted Ind-) of recA and/or the downstream genes was achieved through the placement of transcription terminators within the operon, either upstream or downstream of recA. Prevention of the competence-specific induction of recA severely affected spontaneous transformation. Transformation efficiencies of recA+ (Ind-) and of wild-type cells were compared under various conditions and with different donor DNA. Chromosomal transformation was reduced 17-(chromosomal donor) to 45-fold (recombinant plasmid donor), depending on the donor DNA, and plasmid establishment was reduced 129-fold. Measurement of uptake of radioactively labelled donor DNA in transformed cells in parallel with scoring for transformants (chromosomal donor) revealed normal uptake, but a 21-fold reduction in recombination in a recA+ (Ind-) strain, indicating that the transformation defect was primarily in recombination. Strikingly enough, a much larger (460-fold) reduction in recombination was observed for the shortest homologous donor fragment used (878 nucleotides long). Possible interpretations of the observation that basal RecA appears unable to promote efficient recombination whatever the number and the length of donor fragments taken up are proposed. The role of recA induction is discussed in view of the potential contribution of transformation to genome plasticity in this pathogen.