Expression of two Escherichia coli acetyl-CoA carboxylase subunits is autoregulated.

Acetyl-CoA carboxylase (ACC) catalyzes the first step of fatty acid biosynthesis, the synthesis of malonyl-CoA from acetyl-CoA using ATP and bicarbonate. In Escherichia coli and most other bacteria, ACC is composed of four subunits encoded by accA, accB, accC, and accD. Prior work from this laboratory showed that the in vivo expression of the accBC operon had a strikingly nonlinear response to gene copy number (Li, S.-J, and Cronan, J. E., Jr. (1993) J. Bacteriol. 175, 332-340) in that the presence of 50 or more copies of the accBC operon resulted in only a 2-3-fold increase in AccB and AccC. We now report that AccB functions to negatively regulate transcription of the accBC operon. Expression of a chimeric protein consisting of the N terminus of E. coli AccB and the C-terminal bioinylation domain of Bacillus subtilis AccB down-regulated transcription of the E. coli accBC operon. A truncated form of AccB consisting of the N-terminal 68 amino acids of E. coli AccB was sufficient to negatively regulate the accBC operon. In vivo bypass of acetyl-CoA carboxylase activity by expression of a malonyl-CoA synthase from Rhizobium trifolii allowed construction of strain deleted for the accA and accB genes. Unexpectedly, the deltaaccB mutation could not be resolved from the deltaaccA mutation. Transcription of the accBC operon in the deltaaccB deltaaccA strain continued well into stationary phase under growth conditions that normally result in greatly decreased transcription. These data support a model in which AccB acts as an autoregulator of accBC operon transcription.