Abstract
Our results do not eliminate the possibility that coding sequences, other than UA combinations, could also serve as chain-terminating signals. This is of particular importance with regard to the results reported with poly AUG''s. It is of interest that poly UX does not chainterminate, since this observation suggests that absolute "nonsense" cannot be translated as a chain-terminating signal. All of these data are consistent with the hypothesis that a nucleotide sequence on the messenger RNA plays a functional role in the process of chain completion. Since isolated polylysyl sRNA is stable to incubation with the supernatant fraction, since release of peptidyl sRNA with the poly AU (2:1) system seems to prevent chain completion, and since peptidyl sRNA does not accumulate with other copolymers including f2 RNA or the natural E. coli messengers, we are inclined to believe that cleavage of peptidyl sRNA occurs on the ribosomes. However, further studies are required to substantiate this assumption.