The PyrR protein regulates expression of pyrimidine biosynthetic (pyr) genes in many bacteria. PyrR binds to specific sites in the 5¢ leader RNA of target operons and favors attenuation of transcription. Filter binding and gel mobility assays were used to characterize the binding of PyrR from Bacillus caldolyticusto RNA sequences (binding loops) from the three attenuation regions of theB. caldolyticus pyroperon.