Regulates transcriptional attenuation of the pyrimidine nucleotide (pyr) operon in response to exogenous pyrimidines, by binding to the anti-antiterminator region of the 5' leader on pyr mRNA. This probably favors formation of a transcription terminator hairpin, leading to a reduced expression of downstream genes. Also displays a weak uracil phosphoribosyltransferase activity which is not physiologically significant. diphosphate + UMP = 5-phospho-alpha-D-ribose 1-diphosphate + uracil Optimum pH is 9.2 for UPRTase activity. Homodimer and homohexamer; in equilibrium. Belongs to the purine/pyrimidine phosphoribosyltransferase family. PyrR subfamily.