Genome-wide definition of the SigF regulon in Mycobacterium tuberculosis

In Mycobacterium tuberculosis the alternative sigma factor SigF controls the expression of a particular subset of genes by altering RNA polymerase specificity. Here, we utilize two genome-wide approaches to identify SigF-binding sites: chromatin immunoprecipitation (ChIP-on-chip) and microarray analysis of SigF-mediated transcripts. Since SigF is not an abundant protein in the logarithmic phase of growth, a pristinamyin IA-inducible system was used to control its expression. We identified 67 high-affinity SigF-binding sites and 16 loci where a SigF promoter directs the expression of a transcript. These loci include sigF itself, genes involved in lipid and intermediary metabolism and virulence, and at least one transcriptional regulator (Rv2884), possibly acting downstream of SigF. In addition, SigF was also found to direct the transcription of the gene for small RNA F6. Many loci were also found where SigF may be involved in antisense transcription, and in two cases (Rv1358 and Rv1870c) the SigF-dependent promoter was located within the predicted coding sequence. Quantitative PCR confirmed the microarray findings and 5'-rapid amplification of cDNA ends was used to map the SigF-specific transcriptional start points. A canonical SigF consensus promoter sequence GGTTT-N((15-17))-GGGTA was found prior to 11 genes. Together, these data help to define the SigF regulon and show that SigF not only governs expression of proteins such as the virulence factor, HbhA, but also impacts novel functions, such as noncoding RNAs and antisense transcripts.