Reiterative transcription is a reaction catalyzed by RNA polymerase, in which nucleotides are repetitively added to the 3' end of a nascent transcript due to upstream slippage of the transcript without movement of the DNA template. In Escherichia coli, the expression of several operons is regulated through mechanisms in which high intracellular levels of UTP promote reiterative transcription that adds extra U residues to the 3' end of a nascent transcript during transcription initiation. Immediately following the addition of one or more extra U residues, the nascent transcripts are released from the transcription initiation complex, thereby reducing the level of gene expression. Therefore, gene expression can be regulated by internal UTP levels, which reflect the availability of external pyrimidine sources. The magnitude of gene regulation by these mechanisms varies considerably, even when control mechanisms are analogous. These variations apparently are due to differences in promoter sequences. One of the operons regulated (in part) by UTP-sensitive reiterative transcription in E. coli is the carAB operon, which encodes the first enzyme in the pyrimidine nucleotide biosynthetic pathway. In this study, we used the carAB operon to examine the effects of nucleotide sequence at and near the transcription start site and spacing between the start site and -10 region of the promoter on reiterative transcription and gene regulation. Our results indicate that these variables are important determinants in establishing the extent of reiterative transcription, levels of productive transcription, and range of gene regulation.