Tryptophan Inhibits Proteus vulgaris TnaC Leader Peptide Elongation, Activating tna Operon Expression

Expression of thetna operon ofEscherichia coli and ofProteus vulgaris is induced byl -tryptophan. InE. coli , tryptophan action is dependent on the presence of several critical residues (underlined) in the newly synthesized TnaC leader peptide,W FNID XXL/I XXXXP . These residues are conserved in TnaC ofP. vulgaris and of other bacterial species. TnaC ofP. vulgaris has one additional feature, distinguishing it from TnaC ofE. coli ; it contains two C-terminal lysine residues following the conserved proline residue. In the present study, we investigatedl -tryptophan induction of theP. vulgaris tna operon, transferred on a plasmid intoE. coli . Induction was shown to bel -tryptophan dependent; however, the range of induction was less than that observed for theE. coli tna operon. We compared the genetic organization of both operons and predicted similar folding patterns for their respective leader mRNA segments. However, additional analyses revealed thatl -tryptophan action in theP. vulgaris tna operon involves inhibition of TnaC elongation, following addition of proline, rather than inhibition of leader peptide termination. Our findings also establish that the conserved residues in TnaC ofP. vulgaris are essential forl -tryptophan induction, and for inhibition of peptide elongation. TnaC synthesis is thus an excellent model system for studies of regulation of both peptide termination and peptide elongation, and for studies of ribosome recognition of the features of a nascent peptide.