Mycobacterium leprae hsp18 promoter-EGFP transcriptional fusion construct: Environmental stress and strain-specific expression.

The Hsp18 protein is a major T-cell antigen of Mycobacterium leprae belonging to the family of small heat-shock proteins. The protein is specifically regulated at post-translational level during the intracellular growth of M. leprae within macrophages due to auto-phosphorylation, indicating its importance in the survival of the bacterium. The promoter and regulatory sequences that control hsp18 expression are located within a 256-bp sequence upstream of the translation start site. However, there are no studies describing either characterization of the hsp18 promoter or its genetic regulation. Therefore, we constructed an hsp18-EGFP transcriptional fusion in an E. coli-Mycobacterium shuttle vector. A 168-bp sequence comprising the hsp18 promoter was cloned upstream of the EGFP gene and transformed in M. smegmatis, and the integration of the construct was confirmed by Southern hybridization. hsp18 promoter activity was measured by analyzing EGFP expression in M. smegmatis and Escherichia coli grown under different environmental stress conditions normally encountered by M. leprae in vivo. We found that the 168-bp upstream sequence of hsp18 could function as a promoter, and the regulation of hsp18 expression was host-, environmental stress-, and temperature-dependent. Appreciable EGFP expression was detected in M. smegmatis grown under normal conditions, and theexpression was significantly increased by environmental stress. However, EGFP expression was observed in E. coli only under stress conditions. Comparative sequence analysis revealed the putative sigma factor C (SigC)-binding site within the 168-bp promoter sequence of hsp18, which might be involved in the regulation of hsp18 expression during stress conditions in M. leprae. Thus, our data demonstrated the transcriptional regulation of hsp18 expression in response to different environmental stress conditions, possibly through SigC in Mycobacterium. Further, this shuttle vector could be used for the functional characterization of M. leprae genes in heterologous systems.