Characterization of Rv0093c as a new anti-sigma factor of Mycobacterium tuberculosis

Mycobacterium tuberculosis genome encodes thirteen sigma factors, small subunits of the RNA polymerase that allow rapid adaptation of the bacterium transcriptional landscape, playing a fundamental role in stress-response and pathogenesis. Among them, alternative Sigma Factor C (SigC) is still poorly characterized: its function is seemingly linked to copper uptake and biofilm formation, but its regulon remains undefined. Furthermore, no anti-sigma has been associated to it yet, although the protein Rv0093c represents a suitable candidate. In order to investigate SigC role in the physiology of M. tuberculosis, mutants lacking its structural gene, as well as rv0093c, were generated and analyzed in a number of contexts such as different culture conditions, SigC-related gene expression and biofilm development. Although no significant differences were found between M. tuberculosis wildtype strain and its mutants when cultured in low-copper conditions, a marked sensitivity to high copper concentrations was detected for the mutant lacking rv0093c compared to the wildtype strain. Moreover, genes that had been reported as part of SigC regulon were indeed upregulated in the rv0093c KO mutant, supporting the hypothesis of Rv0093c effectively being SigC-specific anti-sigma factor, and redefying SigC implication in copper metabolism. Evidence of physical interaction between the two proteins was also obtained experimentally, by setting up a mycobacterial protein-fragment complementation assay, executed in the non-pathogenic model species Mycobacterium smegmatis.