Regulates the transcription of the pyrimidine nucleotide (pyr) operon in response to exogenous pyrimidines. Also displays a weak uracil phosphoribosyltransferase activity which is not physiologically significant. diphosphate + UMP = 5-phospho-alpha-D-ribose 1-diphosphate + uracil Belongs to the purine/pyrimidine phosphoribosyltransferase family. PyrR subfamily.