In Escherichia coli, expression of the carAB operon is subject to cumulative repression which occurs by ArgR-mediated repression at a downstream promoter, P2, and by pyrimidine-mediated regulation at an upstream promoter, P1. In this study, we show that pyrimidine-mediated regulation occurs in part through a mechanism involving UTP-sensitive reiterative transcription (i.e., repetitive addition of U residues to the 3' end of a nascent transcript due to transcript-template slippage). In this case, reiterative transcription occurs at the end of a run of three T · A base pairs in the initially transcribed region of the carAB P1 promoter. The sequence of this region is 5'-GTTTGC (nontemplate strand). In the proposed regulatory mechanism, increased intracellular levels of UTP promote reiterative transcription, which results in the synthesis of transcripts with the sequence GUUUU(n) (where n = 1 to >30). These transcripts are not extended downstream to include structural gene sequences. In contrast, lower levels of UTP enhance normal template-directed addition of a G residue at position 5 of the nascent transcript. This addition precludes reiterative transcription and permits normal transcript elongation capable of producing translatable carAB transcripts. Thus, carAB expression, which is necessary for pyrimidine nucleotide (and arginine) biosynthesis, increases in proportion to the cellular need for UTP. The proposed mechanism appears to function independently of a second pyrimidine-mediated control mechanism that involves the regulatory proteins Carp and integration host factor.