Regulation of pyr gene expression in Mycobacterium smegmatis by PyrR-dependent translational repression

Regulation of pyrimidine biosynthetic (pyr) genes by a transcription attenuation mechanism that is mediated by the PyrR mRNA-binding regulatory protein has been demonstrated for numerous gram-positive bacteria. Mycobacterial genomes specify pyrR genes and contain obvious PyrR-binding sequences in the initially transcribed regions of their pyr operons, but transcription antiterminator and attenuation terminator sequences are absent from their pyr 5' leader regions. This work demonstrates that repression of pyr operon expression in Mycobacterium smegmatis by exogenous uracil requires the pyrR gene and the pyr leader RNA sequence for binding of PyrR. Plasmids containing the M. smegmatis pyr promoter-leader region translationally fused to lacZ also displayed pyrR-dependent repression, but transcriptional fusions of the same sequences to a lacZ gene that retained the lacZ ribosome-binding site were not regulated by PyrR plus uracil. We propose that PyrR regulates pyr expression in M. smegmatis, other mycobacteria, and probably in numerous other bacteria by a translational repression mechanism in which nucleotide-regulated binding of PyrR occludes the first ribosome-binding site of the pyr operon.