Activator role of the pneumococcal Mga-like virulence transcriptional regulator.

Global transcriptional regulators that respond to specific environmental signals are crucial in bacterial pathogenesis. In the case of the Gram-positive pathogen Streptococcus pneumoniae (the pneumococcus), the sp1800 gene of the clinical isolate TIGR4 encodes a protein that exhibits homology to the Mga "stand-alone" response regulator of the group A Streptococcus. Such a pneumococcal protein was shown to play a significant role in both nasopharyngeal colonization and development of pneumonia in murine infection models. Moreover, it was shown to repress the expression of several genes located within the rlrA pathogenicity islet. The pneumococcal R6 strain, which derives from the D39 clinical isolate, lacks the rlrA islet but has a gene (here named mga(Spn)) equivalent to the sp1800 gene. In this work, and using in vivo approaches, we have identified the promoter of the mga(Spn) gene (Pmga) and demonstrated that four neighboring open reading frames of unknown function (spr1623 to spr1626) constitute an operon. Transcription of this operon is under the control of two promoters (P1623A and P1623B) that are divergent from the Pmga promoter. Furthermore, we have shown that the Mga(Spn) protein activates the P1623B promoter in vivo. This activation requires sequences located around 50 to 120 nucleotides upstream of the P1623B transcription start site. By DNase I footprinting assays, we have also demonstrated that such a region includes an Mga(Spn) binding site. This is the first report on the activator role of the pneumococcal Mga-like protein.