Mycobacterium tuberculosis rrn promoters: differential usage and growth rate-dependent control

Mycobacterium tuberculosis is a slow-growing pathogen and is characterized by a low content of RNA per unit of DNA. rRNAs represent a major proportion of the total RNA pool, and the entire requirement for rRNA is met by transcription from a single rrn operon that is driven by two promoters, P1 and P3. This study attempted to analyze the specific role of the rrn promoter in determining the characteristically low levels of RNA in M. tuberculosis. For this purpose, the activity of the M. tuberculosis rrn promoter as a function of the growth rate was studied by rrn-lacZ promoter fusion, hybridization, and primer extension analysis in M. smegmatis. rrn promoter signals were faithfully recognized in M. smegmatis cultures harboring the rrn-lacZ promoter construct. In M. smegmatis cultures that displayed doubling times varying between 3.06 and 6.5 h, beta-galactosidase activity increased approximately sixfold in proportion to the growth rate (mu). There was a corresponding increase in the amount of lacZ-specific mRNA, while the plasmid copy number remained essentially unchanged. For any given mu, the P3 promoter was approximately twofold more efficiently utilized than the P1 promoter. Since both promoters of the M. tuberculosis rrn operon are regulatable as a function of growth rate in M. smegmatis cultures, it is implied that the inherent structure or sequence of the rrn promoter per se is not primarily responsible for the observed lack of modulation of RNA synthesis in M. tuberculosis.