The Ayerst Award Lecture 1976. Novel factors in protein biosynthesis.

Comparison of nucleotide sequences surrounding the initiation sites of a number of mRNAs reveals few common features. These may be the presence of in- or out-of-phase nonsense codons and (or) polypurine bases complementary to the 16S RNA of the 30S subunit of ribosomes. Since the bases which precede or follow an initiation site vary in length and composition we have examined whether they play a role as spacers between cistrons or whether they have an active function in the termination and initiation of translation. In vitro we have observed that some sequences 5' terminal to AUG are preferred over others in forming an initiation complex. The same bases have much less effect when present at the 3' terminal end of an AUG codon. When the 5' terminal codon is the termination codon UAA, absolutely no initiation complex can be detected. This suggests that spacing may be needed between a stop and a start codon. Conversely, the hexamer AUGUAA failed to elicit chain termination. This was so in systems that terminated when free UAA was added or when a sense triplet was present between the initiation and termination triplets. These results suggest that ribosomes may recognize the stop triplet. Hence ribosomes may not obey simple A and P site models in the termination reaction.