The Enterococcus faecalis pyr operon is regulated by autogenous transcriptional attenuation at a single site in the 5' leader.

The 5' end of the Enterococcus faecalis pyr operon specifies, in order, the promoter, a 5' untranslated leader, the pyrR gene encoding the regulatory protein for the operon, a 39-nucleotide (nt) intercistronic region, the pyrP gene encoding a uracil permease, a 13-nt intercistronic region, and the pyrB gene encoding aspartate transcarbamylase. The 5' leader RNA is capable of forming stem-loop structures involved in attenuation control of the operon. No attenuation regions, such as those found in the Bacillus subtilis pyr operon, are present in the pyrR-pyrP or pyrP-pyrB intercistronic regions. Several lines of evidence demonstrate that the E. faecalis pyr operon is repressed by uracil via transcriptional attenuation at the single 5' leader termination site and that attenuation is mediated by the PyrR protein.