The Evolution of Mycobacterium tuberculosis: The Mycobacterial SigK-RskA Regulatory System.

摘要

The mycobacteria genus comprises bacteria pathogenic for humans and animals, including M. tuberculosis complex (MTC). In the past decade, considerable research has focused on genomic studies differences between MTC strains and sub-species. These data largely address the devolution of MTC organisms, through a process of reductive genomics and single nucleotide polymorphisms. We hypothesized that these findings present an incomplete evolutionary scenario for the pathogen M. tuberculosis. To test this possibility, we have conducted comparative genomic analysis among sequenced mycobacteria to look for evidence of horizontal gene transfers during the evolution of the genus. Using a bioinformatic screen, we predicted a number of foreign genes acquisitions during the step-wise genesis of M. tuberculosis . Along with others, this study emphasizes another side of the evolution of M. tuberculosis and demonstrates that before the process of genomic decay, M. tuberculosis had acquired foreign genes.;Over-all, this study has provided new and complementary understanding on M. tuberculosis evolution. Additionally, our evolutionary approach has contributed to a better understanding of two major areas of research, specifically Sigma factor signaling and M. tuberculosis pathogenesis.;The description of the M. tuberculosis evolution is strengthened when accompanied by functional characterization of these evolutionary events. Therefore, we have focused on an already-known example of micro-evolution to determine its role in M. tuberculosis biology. Our laboratory had previously established that MTC members exhibit variable production of eleven proteins due to mutations in the anti-sigma factor of SigK (RskA). As a consequence, in M. tuberculosis, their expression is low in vitro but strongly induced during infection; in contrast, M. bovis and the Oryx bacillus constitutively express these proteins. We first determined which genes are SigK-regulated and described the SigK promoters using luciferase technology. We then used these tools for a detailed study of the function of RskA from different MTC organisms. These experiments demonstrated that the anti-sigma factor RskA is not only an inhibitor of SigK but also presents an activator function. Finally, the activating property of RskA was used to generate M. tuberculosis strains over-expressing the SigK regulon, enabling us to test the effect of over-expression on the host pathogen relationship during in vivo infections. These experiments reproducibly demonstrated that over-expressing SigK-regulated genes results in increased bacterial dissemination from the site of infection along with increased survival of the host.