TRANSLATIONAL TERMINATION EFFICIENCY IN BOTH BACTERIA AND MAMMALS IS REGULATED BY THE BASE FOLLOWING THE STOP CODON

The translational stop signal and polypeptide release factor (RF) comp lexed with Escherichia coli ribosomes have been shown to be in close p hysical contact by site-directed photochemical cross-linking experimen ts. The RF has a protease-sensitive site in a highly conserved exposed loop that is proposed to interact with the peptidyltransferase center of the ribosome. Loss of peptidyl-tRNA hydrolysis activity and enhanc ed codon-ribosome binding by the cleaved RF is consistent with a model whereby the RF spans the decoding and peptidyltransferase centers of the ribosome with domains of the RF linked by conformational coupling. The cross-link between the stop signal and RF at the ribosomal decodi ng site is influenced by the base following the termination codon. Thi s base determines the efficiency with which the stop signal is decoded by the RF in both mammalian and bacterial systems in vivo. The wide r ange of efficiencies correlates with the frequency with which the sign als occur at natural termination sites, with rarely used weak signals often found at recoding sites and strong signals found in highly expre ssed genes. Stop signals are found at some recoding sites in viruses w here -1 frame-shifting occurs, but the generally accepted mechanism of simultaneous slippage from the A and P sites does not explain their p resence here. The HIV-1 gag-pol -1 frame shifting site has been used t o show that stop signals significantly influence frame-shifting effici ency on prokaryotic ribosomes by a RF-mediated mechanism. These data c an be explained by an E/P site simultaneous slippage mechanism whereby the stop codon actually enters the ribosomal A site and can influence the event.