Rv1255c, a dormancy-related transcriptional regulator of TetR family in Mycobacterium tuberculosis, enhances isoniazid tolerance in Mycobacterium smegmatis.

Mycobacterium tuberculosis is exposed to diverse stresses inside the host during dormancy. Meanwhile, many metabolic and transcriptional regulatory changes occur, resulting in physiological modifications that help M. tuberculosis to adapt to these stresses. The same physiological changes also cause antibiotic tolerance in dormant M. tuberculosis. However, the transcriptional regulatory mechanism of antibiotic tolerance during dormancy remains unclear. Here, we showed that the expression of Rv1255c, an uncharacterised member of the tetracycline repressor family of transcriptional regulators, is upregulated during different stresses and hypoxia-induced dormancy. Antibiotic tolerance and efflux activities of Mycobacterium smegmatis constitutively expressing Rv1255c were analysed, and interestingly, it showed increased isoniazid tolerance and efflux activity. The intrabacterial isoniazid concentrations were found to be low in M. smegmatis expressing Rv1255c. Moreover, orthologs of the M. tuberculosis katG, gene of the enzyme which activates the first-line prodrug isoniazid, are overexpressed in this strain. Structural analysis of isoforms of KatG enzymes in M. smegmatis identified major amino acid substitutions associated with isoniazid resistance. Thus, we showed that Rv1255c helps M. smegmatis tolerate isoniazid by orchestrating drug efflux machinery. In addition, we showed that Rv1255c also causes overexpression of katG isoform in M. smegmatis which has amino acid substitutions as found in isoniazid-resistant katG in M. tuberculosis.