[Characterization of toxin-antitoxin systems in Mycobacterium tuberculosis]

To characterize the toxin-antitoxin system (TA system) in Mycobacterium tuberculosis, which consist of MazF homologue gene and its upstream gene.

Seven M. tuberculosis MazF homologues were induced alone or co-expressed with their upstream genes respectively in Escherichia coli and Mycobacterium smegmatis, to test the toxic effects of the MazF homologues on bacteria growth, and the antitoxic effects of protein encoded by their upstream genes. The RNA cleavage activity of MazF homologous was identified in vitro with Rv0707 mRNA as the substrate. The promoter region of the identified toxin-antitoxin loci in M. tuberculosis was cloned in front of the lacZ reporter gene in pSD5B vector. The promoter activity was measured under the normal or starvation condition.

The growth of either E. coli or M. smegmatis was inhibited by four MazF homologous proteins, among which Rv1102c, Rv1991c and Rv2801c, but not mtPemK, had the RNA cleavage activities. The toxic effects and RNA cleavage activities of Rv1102c, Rv1991c and Rv2801c were inhibited by their corresponding antitoxin Rv1103c, Rv1991a and Rv2801a, respectively. The other three MazF homologues, Rv1942c, Rv0659c and Rv1495, were not toxic to E. coli and M. smegmatis and also could not cleave RNA. It was found that the promoter activities of Rv2801a-2801c and Rv1991a-1991c systems were significantly increased under the complete starvation condition.

Our results demonstrated that Rv1103c-1102c, Rv1991a-1991c, Rv2801a-2801c and mtPemI-mtPemK were typical toxin-antitoxin systems in M. tuberculosis. Rv1102c, Rv1991c and Rv2801c were toxin proteins which inhibited cell growth through their RNA cleavage activities, while the mechanism of mtPemK toxin is still unknown. It is possible that Rv2801a-2801c and Rv1991a-1991c systems are involved in the starvation stress response.