| Summary: | <p>Abstract</p> <p>Background</p> <p><it>mce3 </it>is one of the four virulence-related <it>mce </it>operons of <it>Mycobacterium tuberculosis</it>. In a previous work we showed that the overexpression of Mce3R in <it>Mycobacterium smegmatis </it>and <it>M. tuberculosis </it>abolishes the expression of <it>lacZ </it>fused to the <it>mce3 </it>promoter, indicating that Mce3R represses <it>mce3 </it>transcription.</p> <p>Results</p> <p>We obtained a knockout mutant strain of <it>M. tuberculosis </it>H37Rv by inserting a hygromycin cassette into the <it>mce3R </it>gene. The mutation results in a significant increase in the expression of <it>mce3 </it>genes either <it>in vitro </it>or in a murine cell macrophages line as it was determined using promoter-<it>lacZ </it>fusions in <it>M. tuberculosis</it>. The abundance of <it>mce1</it>, <it>mce2 </it>and <it>mce4 </it>mRNAs was not affected by this mutation as it was demonstrated by quantitative RT-PCR. The <it>mce3R </it>promoter activity in the presence of Mce3R was significantly reduced compared with that in the absence of the regulator, during the <it>in vitro </it>culture of <it>M. tuberculosis</it>.</p> <p>Conclusion</p> <p>Mce3R repress the transcription of <it>mce3 </it>operon and self regulates its own expression but does not affect the transcription of <it>mce1</it>, <it>mce2 </it>and <it>mce4 </it>operons of <it>M. tuberculosis</it>.</p>
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