A tale of two functions: enzymatic activity and translational repression by carboxyltransferase

Acetyl-CoA Carboxylase catalyzes the first commit-ted step in fatty acid synthesis. Escherichia coli acetyl-CoA carboxylase is composed of biotin carboxylase, carboxyltransferase and biotin carboxyl carrier protein functions. The accA and accD genes that code for the a- and b-subunits, respectively, are not in an operon, yet yield an a2b2 carboxyltransferase. Here, we report that carboxyltransferase regulates its own translation by binding the mRNA encoding its subunits. This interaction is mediated by a zinc finger on the b-subunit; mutation of the four cysteines to alanine diminished nucleic acid binding and catalytic activity. Carboxyltransferase binds the coding regions of both subunit mRNAs and inhibits transla-tion, an inhibition that is relieved by the substrate acetyl-CoA. mRNA binding reciprocally inhibits catalytic activity. Preferential binding of carboxyl-transferase to RNA in situ was shown using fluorescence resonance energy transfer. We propose an unusual regulatory mechanism by which carboxyltransferase acts as a ‘dimmer switch ’ to regulate protein production and catalytic activity, while sensing the metabolic state of the cell through acetyl-CoA concentration