Examination of the SigC regulon and cobalamin biosynthesis in Mycobacterium tuberculosis
Grosse-Siestrup, Benjamin Tobias
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The causative agent of tuberculosis (TB), Mycobacterium tuberculosis, remains a leading cause of morbidity and mortality, claiming ~2 million lives annually. The current live-attenuated vaccine protects against meningeal TB in children, but protection against pulmonary TB varies from 0-80% among post-adolescents. Understanding the mechanisms by which M. tuberculosis adapts to successfully reside and cause disease in the host is vital for rational design of therapeutics, vaccines, and diagnostics. In M. tuberculosis, a transcription initiation protein, sigma factor C (SigC), is a known virulence factor in murine and guinea pig TB infection models. However, no phenotypes in vitro have been reported. How SigC is regulated and how it aids the pathogen in vivo is unknown. In this study, the regulon of genes controlled by SigC was examined. Use of M. tuberculosis strains either lacking or overexpressing the sigC gene has revealed a role for SigC in trace metal acquisition. Use of medium deficient in trace metals resulted in a growth defect by a sigC mutant and will aid future studies to explore the regulation of SigC. This sigC mutant was confirmed to be attenuated in immune-compromised mice. The genomic location of the monocystronic sigC gene within a cluster of potential cobalamin biosynthesis genes led to our interest in cobalamin production. Cobalamins, such as vitamin B12, function as cofactors in various metabolic processes. While studies over fifty years ago reported vitamin B12 synthesis by mycobacteria, recent studies suggest that it is not made by M. tuberculosis. To investigate this discrepancy, production of cobalamin in mycobacteria was re-examined. Interestingly, B12 was not detected in any of the species that cause tuberculosis in mammals, but was produced in all other species tested. The presence of a full complement of cobalamin biosynthetic genes in M. tuberculosis suggests this pathogen produces the cofactor, but not under the conditions assayed. Differences in acquisition of trace metals for cobalamin synthesis by mammalian versus environmental mycobacteria or limitations of the detection methods utilized may account for the observed differences. Research is underway in the laboratory to test these hypotheses.