Autoregulation of the tufB operon in Salmonella

In Salmonella enterica and related species, translation elongation factor EF-Tu is encoded by two widely separated but near-identical genes, tufA and tufB. Two thirds of EF-Tu is expressed from tufA with the remaining one third coming from tufB. Inactivation of tufA is partly compensated by a doubling in the amount of EF-TuB but the mechanism of this up-regulation is unknown. By experimental evolution selecting for improved growth rate in a strain with an inactive tufA we selected six different noncoding or synonymous point mutations close to the tufB start codon. Based on these results we constructed a total of 161 different point mutations around the tufB start codon, as well as tufB 3'-truncations, and measured tufB expression using tufB-yfp transcriptional and translational fusions. The expression data support the presence of two competing stem-loop structures that can form in the 5'-end of the tufB mRNA. Formation of the 'closed' structure leads to Rho-dependent transcriptional termination of the tufB mRNA. We propose a model in which translational speed is used as a sensor for EF-Tu concentration and where the expression of tufB is post-transcriptionally regulated. This model describes for the first time how expression of the most abundant Salmonella protein is autoregulated.