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The ESX-1 secretion system ofMycobacterium tuberculosis has to be precisely regulated since the secreted proteins, although required for a successful virulent infection, are highly antigenic and their continued secretion would alert the immune system to the infection. The transcription of a five-gene operon containingespACD-Rv3613c-Rv3612c , which is required for ESX-1 secretion and is essential for virulence, was shown to be positively regulated by the EspR transcription factor. Thus, transcription from the start site, found to be located 67 bp upstream ofespA , was dependent upon EspR enhancer-like sequences far upstream (between 884 and 1,004 bp), which we term thee spA a ctivatingr egion (EAR). The EAR contains one of the known binding sites for EspR, providing the firstin vivo evidence that transcriptional activation at theespA promoter occurs by EspR binding to the EAR and looping out DNA between this site and the promoter. Regulation of transcription of this operon thus takes place over long regions of the chromosome. This regulation may differ in some members of theM. tuberculosis complex, includingMycobacterium bovis , since deletions of the intergenic region have removed the upstream sequence containing the EAR, resulting in loweredespA expression. Consequent differences in expression of ESX-1 in these bacteria may contribute to their various pathologies and host ranges. The virulence-critical nature of this operon means that transcription factors controlling its expression are possible drug targets.

Long-Range Transcriptional Control of an Operon Necessary for Virulence-Critical ESX-1 Secretion in Mycobacterium tuberculosis