Regulates transcriptional attenuation of the pyrimidine nucleotide (pyr) operon by binding in a uridine-dependent manner to specific sites on pyr mRNA. This disrupts an antiterminator hairpin in the RNA and favors formation of a downstream transcription terminator, 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 Homodimer and homohexamer; in equilibrium. Belongs to the purine/pyrimidine phosphoribosyltransferase family. PyrR subfamily.