Transcriptional attenuation of the Bacillus subtilis pyr operon by the PyrR regulatory protein and uridine nucleotides in vitro.

Transcriptional attenuation of the pyrimidine biosynthetic (pyr) operon from Bacillus subtilis was reconstituted with an in vitro system that consisted of pyr DNA templates, B. subtilis RNA polymerase, four ribonucleoside triphosphates, and the purified B. subtilis PyrR regulatory protein. The templates used each specified one of the three known attenuation regions of the pyr operon. Runoff (read-though) and terminated transcripts of the predicted lengths were the only major products synthesized. Transcription of the template that specifies the 5' leader attenuation region of the operon was examined in detail. Termination of transcription at the attenuator was strongly promoted by the combination of PyrR plus UMP. The concentration of UMP required for half-maximal effect was 2.5 microM. UTP also promoted termination in the presence of PyrR, but concentrations 10-fold higher than UMP were required; UDP was only effective at 100 times the concentration of UMP. Other pyrimidine and purine metabolites tested did not affect termination. PRPP, which like UMP is a substrate for the uracil phosphoribosyltransferase activity of PyrR, antagonized UMP-dependent transcriptional termination, but uracil did not. Transcriptional attenuation by PyrR plus UMP was also demonstrated in vitro with templates from the other two pyr attenuation regions. The results strongly support the model for transcriptional regulation of the B. subtilis pyr operon previously proposed by R. J. Turner, Y. Lu, and R. L. Switzer (J. Bacteriol. 176:3708-3722, 1994).