The PyrR binding site motif is an RNA element that was found upstream of a variety of genes involved in pyrimidine biosynthesis and transport in Bacillus subtilis and related Gram-positive bacteria from the Firmicutes phylum. The PyrR RNA element is a binding site of the PyrR regulatory protein that binds the leader RNA regions of the PyrR regulon target genes in the presence of uridine nucleotides . UMP or UDP nucleotides are less effective co-regulators of PyrR in comparison with UTP. The PyrR protein binds an RNA-binding loop of the PyrR RNA element, allowing a terminator hairpin to form and repressing the downstream genes .
The minimal RNA that bound tightly to PyrR was 28 nt long. PyrR likely recognizes conserved RNA sequences, but only if they are properly positioned in the correct secondary structure. The PyrR regulon in B. subtilis is known to include three operons, the pyrimidine biosynthesis operon pyrBC-carAB-pyKDFE, the uracile permease gene pyrP, and the regulator gene pyrR [17322189