Instructing the translating ribosome to sense l-tryptophan during synthesis of the TnaC nascent regulatory peptide

Abstract

The proteins specified by the tnaCAB operon, an operon present in Escherichia coli and other bacterial species, participate in several important reactions within the bacterial cell. Transcription of this operon is generally controlled by two mechanisms: by catabolite repression, and by an attenuation mechanism involving the inhibition of ribosome function by sensing the l-tryptophan level in the cell. Interactions between the TnaC nascent regulatory peptide and features of the ribosome peptide exit tunnel lead to the creation of a l-tryptophan-binding site within the ribosome. Once bound, l-tryptophan blocks the hydrolysis of TnaC-peptidyl-tRNA induced by release factor 2. This action stalls the translating ribosome at the end of the tnaC mRNA open reading frame, inhibiting the subsequent action of the Rho termination factor; this allows transcription of the tnaA and tnaB structural genes to proceed. These genes encode the enzymes tryptophanase and a l-tryptophan transporter, respectively. Understanding how l-tryptophan interacts with the ribosome, thus preventing transcription of the tnaA-tnaB region, which is an objective of our studies, would improve our understanding of this mechanism of gene regulation.