Expression of the pyrBI operon of Escherichia coli K-12, which encodes the subunits of the pyrimidine biosynthetic enzyme aspartate transcarbamylase, is negatively regulated over a several-hundredfold range by pyrimidine availability. This regulation occurs, at least in large part, through a UTP-sensitive attenuation control mechanism in which transcriptional termination at the pyrBI attenuator, a rho-independent transcriptional terminator located immediately upstream of the pyrB structural gene, is regulated by the relative rates of transcription and translation within the pyrBI leader region. There is suggestive evidence that an additional, attenuator-independent control mechanism also contributes to this regulation. To measure the level of regulation that occurs through the attenuation and attenuator-independent control mechanisms, we constructed a mutant strain in which a 9-base-pair deletion was introduced into the attenuator of the chromosomal pyrBI operon. This deletion, which removes the run of thymidine residues at the end of the attenuator, completely abolishes rho-independent transcriptional termination activity. When the mutant strain was grown under conditions of pyrimidine excess, the level of operon expression was 51-fold greater than that of an isogenic pyrBI+ strain. Under conditions of pyrimidine limitation, operon expression was increased an additional 6.5-fold in the mutant. These results demonstrate that the attenuation control mechanism is primarily responsible for pyrimidine-mediated regulation but that there is a significant contribution by an attenuator-independent control mechanism.