Expression of different-size transcripts from the clpP-clpX operon of Escherichia coli during carbon deprivation.

Transcription of the clpP-clpX operon of Escherichia coli leads to the production of two different sizes of transcripts. In log phase, the level of the longer transcript is higher than the level of the shorter transcript. Soon after the onset of carbon starvation, the level of the shorter transcript increases significantly, and the level of the longer transcript decreases. The longer transcript consists of the entire clpP-clpX operon, whereas the shorter transcript contains the entire clpP gene but none of the clpX coding sequence. The RpoH protein is required for the increase in the level of the shorter transcript during carbon starvation. Primer extension experiments suggest that there is increased usage of the sigma(32)-dependent promoter of the clpP-clpX operon within 15 min after the start of carbon starvation. Expression of the clpP-clpX operon from the promoters upstream of the clpP gene decreases to a very low level by 20 min after the onset of carbon starvation. Various pieces of evidence suggest, though they do not conclusively prove, that production of the shorter transcript may involve premature termination of the longer transcript. The half-life of the shorter transcript is much less than that of the longer transcript during carbon starvation. E. coli rpoB mutations that affect transcription termination efficiency alter the ratio of the shorter clpP-clpX transcript to the longer transcript. The E. coli rpoB3595 mutant, with an RNA polymerase that terminates transcription with lower efficiency than the wild type, accumulates a lower percentage of the shorter transcript during carbon starvation than does the isogenic wild-type strain. In contrast, the rpoB8 mutant, with an RNA polymerase that terminates transcription with higher efficiency than the wild type, produces a higher percentage of the shorter clpP-clpX transcript when E. coli is in log phase. These and other data are consistent with the hypothesis that the shorter transcript results from premature transcription termination during production of the longer transcript.