Sequestration of specific tRNA species cognate to the last sense codon of an overproduced gratuitous protein

High-level expression of non-functional model proteins, derived from elonga tion factor EF-Tu by the deletion of an essential domain, greatly inhibits the growth of Escherichia coli partly deficient in peptidyl-tRNA hydrolase. High-level expression in wild-type cells has little effect on growth. The inhibitory effect is therefore presumably due to the sequestration of essen tial tRNA species, partly in the form of free peptidyl-tRNA, The growth inh ibitory effect can be modulated by changing the last sense codon in the gen es encoding the model proteins. Thus, replacement of Ser by Lys or His at t his position increases growth inhibition. The effects of 11 changes studied are related to the rates of accumulation previously observed of the corres ponding families of peptidyl-tRNA. Two non-exclusive hypotheses are propose d to account for these observations: first, the last sense codon of mRNA is a preferred site of peptidyl-tRNA drop-off in cells, due to the slow rate of translation termination compared with sense codon translation; secondly, the relatively long pause of the ribosome at the stop codon (of the order of 1 s), results in significant temporary sequestration on the ribosome of the tRNA cognate to the last sense codon.