Regulation of hmp gene transcription in Mycobacterium tuberculosis: Effects of oxygen limitation and nitrosative and oxidative stress

The Mycobacterium tuberculosis hmp gene encodes a protein which is homologo us to flavohemoglobin in Escherichia coli. Northern blotting analysis demon strated that hmp transcription increased when a microaerophilic culture bec ame oxygen limited as it entered stationary phase at 20 days. There was a f ivefold increase of the hmp transcripts during early stationary phase compa red with the value which was observed in the exponential growth phase. This induction of hmp transcription was not due to changes in the mRNA stabilit y since the half-life of hmp mRNA was very short in a 20-day microaerophili c culture. No induction of hmp mRNA was observed during entry into stationa ry phase when the culture was continuously aerated. hmp transcription was i nduced after a short exposure of a late-exponential-phase culture to anaero bic conditions. These data indicate that oxygen limitation is the trigger f or hmp gene transcription. In addition, when a microaerophilic culture ente red into the stationary phase at 20 days, transcription of hmp increased to a small extent after exposure to S-nitrosoglutathione (a nitric oxide [NO] releaser) and sodium nitroprusside (an NO+ donor) and decreased after expo sure to paraquat (a superoxide generator) and H2O2. In log phase (4 days) a nd late stationary phase (40 days), the transcription of hmp was unaffected by nitrosative and oxidative stress. Three primer extension products were observed. The -10 region is 100% identical to that of promoter T3 in mycoba cteria and shows a strong similarity to the -10 sequence of hmp and rpoS pr omoters in E. coli. These observations of hmp mRNA induction in response to O-2 limitation and nitrosative stress suggest that the hmp gene of M. tube rculosis may have a role in protection of the organism from NO killing unde r microaerophilic conditions.