Convergence of regulatory networks on the pilus locus of Streptococcus pneumoniae

The rlrA pilus locus of Streptococcus pneumoniae is an example of a pathogenicity island acquired through genetic recombination. Many acquired genetic elements commandeer preexisting networks of the new organism for transcriptional regulation. We hypothesized that the rlrA locus has integrated into transcriptional regulatory networks controlling expression of virulence factors important in adhesion and invasion. To test this hypothesis, we determined the impact on pilus expression of known regulators controlling adherence, including the two-component systems CbpR/S and HK/RR03 and the transcriptional regulators of divalent cation transporters MerR and PsaR in vitro and in vivo. It was determined that the pilus locus is down-regulated by preexisting networks designed for adhesion and cation transport/response and that its regulation occurs through RlrA. The pilus locus was found to participate in invasion specifically restricted to lung epithelial cells in vitro. While expression of pili had only a small effect on virulence with an intranasal infection model, pili were critically important with an intratracheal infection model. Thus, expression of pili appears to have become integrated into the regulatory circuits for lung-specific invasion by pneumococci.