Responses ofMycobacterium tuberculosisHemoglobin Promoters to In Vitro and In Vivo Growth Conditions

The success ofMycobacterium tuberculosisas one of the dreaded human pathogens lies in its ability to utilize different defense mechanisms in response to the varied environmental challenges during the course of its intracellular infection, latency, and reactivation cycle. Truncated hemoglobins trHbN and trHbO are thought to play pivotal roles in the cellular metabolism of this organism during stress and hypoxia. To delineate the genetic regulation of theM. tuberculosishemoglobins, transcriptional fusions of the promoters of theglbNandglbOgenes with green fluorescent protein were constructed, and their responses were monitored inMycobacterium smegmatisandM. tuberculosisH37Ra exposed to environmental stresses in vitro and inM. tuberculosisH37Ra after in vivo growth inside macrophages. TheglbNpromoter activity increased substantially during stationary phase and was nearly 3- to 3.5-fold higher than the activity of theglbOpromoter, which remained more or less constant during different growth phases inM. smegmatis, as well as inM. tuberculosisH37Ra. In both mycobacterial hosts, theglbNpromoter activity was induced 1.5- to 2-fold by the general nitrosative stress inducer, nitrite, as well as the NO releaser, sodium nitroprusside (SNP). TheglbOpromoter was more responsive to nitrite than to SNP, although the overall increase in its activity was much less than that of theglbNpromoter. Additionally, theglbNpromoter remained insensitive to the oxidative stress generated by H2O2, but theglbOpromoter activity increased nearly 1.5-fold under similar conditions, suggesting that the trHb gene promoters are regulated differently under nitrosative and oxidative stress conditions. In contrast, transition metal-induced hypoxia enhanced the activity of both theglbNandglbOpromoters at all growth phases; theglbOpromoter was induced ∼2.3-fold, which was found to be the highest value for this promoter under all the conditions evaluated. Addition of iron along with nickel reversed the induction in both cases. Interestingly, a concentration-dependent decrease in the activity of both trHb gene promoters was observed when the levels of iron in the growth media were depleted by addition of an iron chelator. These results suggested that an iron/heme-containing oxygen sensor is involved in the modulation of the trHb gene promoter activities directly or indirectly in conjunction with other cellular factors. The modes of promoter regulation under different physiological conditions were found to be similar for the trHbs in bothM. smegmatisandM. tuberculosisH37Ra, indicating that the promoters might be regulated by components that are common to the two systems. Confocal microscopy of THP-1 macrophages infected withM. tuberculosiscarrying the trHb gene promoter fusions showed that there was a significant level of promoter activity during intracellular growth in macrophages. Time course evaluation of the promoter activity after various times up to 48 h by fluorescence-activated cell sorting analysis of the intracellularM. tuberculosiscells indicated that theglbNpromoter was active at all time points assessed, whereas the activity of theglbOpromoter remained at a steady-state level up to 24 h postinfection and increased ∼2-fold after 48 h of infection. Thus, the overall regulation pattern of theM. tuberculosistrHb gene promoters correlates not only with the stresses that the tubercle bacillus is likely to encounter once it is in the macrophage environment but also with our current knowledge of their functions. The in vivo studies that demonstrated for the first time expression of trHbs during macrophage infection ofM. tuberculosisstrongly indicate that the hemoglobins are required, and thus important, during the intracellular phase of the bacterial cycle. The present study of transcriptional regulation ofM. tuberculosishemoglobins in vitro under various stress conditions and in vivo after macrophage infection supports the hypothesis that biosynthesis of both trHbs (trHbN and trHbO) in the native host is regulated via the environmental signals that the tubercle bacillus receives during macrophage infection and growth in its human host.