| Summary: | <p>Abstract</p> <p>Background</p> <p>The ESAT-6 (early secreted antigenic target, 6 kDa) family collects small mycobacterial proteins secreted by <it>Mycobacterium tuberculosis</it>, particularly in the early phase of growth. There are 23 ESAT-6 family members in <it>M. tuberculosis </it>H37Rv. In a previous work, we identified the Zur- dependent regulation of five proteins of the ESAT-6/CFP-10 family (<it>esxG</it>, <it>esxH</it>, <it>esxQ</it>, <it>esxR</it>, and <it>esxS</it>). <it>esxG </it>and <it>esxH </it>are part of ESAT-6 cluster 3, whose expression was already known to be induced by iron starvation.</p> <p>Results</p> <p>In this research, we performed EMSA experiments and transcriptional analysis of ESAT-6 cluster 3 in <it>Mycobacterium smegmatis </it>(<it>msmeg0615</it>-<it>msmeg0625</it>) and <it>M. tuberculosis</it>. In contrast to what we had observed in <it>M. tuberculosis</it>, we found that in <it>M. smegmatis </it>ESAT-6 cluster 3 responds only to iron and not to zinc. In both organisms we identified an internal promoter, a finding which suggests the presence of two transcriptional units and, by consequence, a differential expression of cluster 3 genes. We compared the expression of <it>msmeg0615 </it>and <it>msmeg0620 </it>in different growth and stress conditions by means of relative quantitative PCR. The expression of <it>msmeg0615 </it>and <it>msmeg0620 </it>genes was essentially similar; they appeared to be repressed in most of the tested conditions, with the exception of acid stress (pH 4.2) where <it>msmeg0615 </it>was about 4-fold induced, while <it>msmeg0620 </it>was repressed. Analysis revealed that in acid stress conditions <it>M. tuberculosis rv0282 </it>gene was 3-fold induced too, while <it>rv0287 </it>induction was almost insignificant.</p> <p>Conclusion</p> <p>In contrast with what has been reported for <it>M. tuberculosis</it>, our results suggest that in <it>M. smegmatis </it>only IdeR-dependent regulation is retained, while zinc has no effect on gene expression. The role of cluster 3 in <it>M. tuberculosis </it>virulence is still to be defined; however, iron- and zinc-dependent expression strongly suggests that cluster 3 is highly expressed in the infective process, and that the cluster contributes to the antigenic profile during the course of infection. Moreover, cluster 3 induction in acid stress conditions strengthens the hypothesis that cluster 3 is expressed in the course of infection.</p> <p>In <it>M. smegmatis</it>, the expression of <it>msmeg0615 </it>and <it>msmeg0620 </it>genes is broadly similar in differing growth phases and in stress conditions, with the exception of acid stress (pH 4.2). Differences in expression between cluster 3 genes can be explained by the presence of internal promoters, both in <it>M. smegmatis </it>and <it>M. tuberculosis</it>.</p>
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