Exposure to antibiotics induces expression of the Mycobacterium tuberculosis sigF gene: Implications for chemotherapy against mycobacterial persistors

The sigF gene encodes an alternate sigma factor found in Mycobacterium tube rculosis and related pathogenic mycobacteria, Determination of conditions o f sigF expression is an important step in understanding the conditional gen e regulation which may govern such processes as virulence and dormancy in m ycobacteria. We constructed an in-frame translational lacZ-kan fusion withi n the sigF gene to determine the conditions of sigF expression. This report er construct was expressed from a multicopy plasmid in a strain of BCG harb oring an integrated luciferase reporter gene under the control of the mycob acteriophage L5 gp71 promoter, Antibiotic exposure, in particular, ethambut ol, rifampin, streptomycin, and cycloserine treatment, increased the level of SigF reporter specific expression in a dose-dependent fashion. The level of SigF reporter specific expression increased over 100-fold in late-stati onary-phase growth compared to that in exponential growth. During the expon ential phase, SigF specific expression could be induced by a number of othe r stresses. Anaerobic metabolism induced SigF by greater than 150-fold, par ticularly in the presence of metronidazole, Cold shock increased the level of SigF specific expression, while heat shock decreased it. Oxidative stres s was also an important inducer of SigF specific expression; a greater indu ction was seen with cumene hydroperoxide than with hydrogen peroxide, Compa risons of bacterial viability as determined by the luciferase assay or by p lating serial dilutions revealed that luciferase gp71-dependent activity wa s an unreliable predictor of the numbers of CFU during stationary-phase gro wth and anaerobic metabolism. The induction of sigF following antibiotic ex posure suggests that this bacterial transcription factor may control genes which are important for mycobacterial persistence in the host during chemot herapy.