Studies on transcriptional regulation of pyruvate formate lyase in Streptococcus pneumoniae

In the nasopharynx, Streptococcus pneumoniae relies on mucin glycoproteins, which are rich in galactose. Galactose stimulates mixed-acid fermentation in S. pneumoniae. Pyruvate formate lyase (PFL) is a key enzyme in mixed-acid fermentation, and its inactivation attenuates pneumococcal virulence, and abolishes mixed-acid fermentation. Limited information is available on the transcriptional regulation of pflA, which codes for pyruvate formate lyase activating enzyme (PFL-AE) and post-translationally activates inactive PFL, and pflB, both required for active PFL. Through analysis of a strain mutated in pflB by microarray, six transcriptional regulators potentially involved in the regulation of pflB were identified. Inactivation of these transcriptional regulators led to decreased growth in the presence of glucose or galactose, and an altered profile of fermentation end products. Using different experimental techniques such as electrophoretic mobility shift assay, and transcriptional lacZ-fusions, three transcriptional regulators were found to be directly controlling the transcription of pflA or pflB. The results show that the catabolite control protein A (CcpA) plays an activating role in pflB regulation in the presence of galactose, whereas the transcriptional regulators MerR2 and GntR repress the transcription of pflB. Moreover, CcpA and MerR2 were determined as the transcriptional repressors of pflA. It was also found that MerR2 contributes to the regulation of ccpA, and that MerR2 and CcpA interact for regulation of pflA and pflB. The activating role of CcpA and MerR2 in the regulation of PlcR, regulator of virulence genes in Bacillus spp., was also determined. Transcriptional lacZ-fusions to the promoters of the transcriptional regulators CcpA, MerR2, and PlcR showed high induction in host-derived sugars such as galactose, glucose, mannose, and N-acetylglucosamine. Loss of CcpA and/or MerR2 had a significant impact on pneumococcal virulence and colonisation in a mouse model of pneumococcal infection.