Analysis of ribosome binding sites from the s1 message of reovirus: Initiation at the first and second AUG codons

Abstract

Two ribosome-protected initiation sites from the s1 message of reovirus have been characterized. Comparison of these sites with the previously determined sequence of s1 mRNA (Li et al., 1980) reveals that wheat germ ribosomes select and protect the first two AUG triplets in that message. This is unusual, since ribosomes initiate at a single site, the 5′-proximal AUG, in almost all other eukaryotic messenger RNAs that have been examined. The first AUG codon in s1 mRNA is preceded by a pyrimidine in position −3, thus distinguishing it from most other eukaryotic messages, which have a purine (usually A) in that position. The behavior of s1 mRNA is consistent with the hypothesis that flanking nucleotides modulate the efficiency with which the migrating 40 S ribosomal subunit recognizes an AUG codon as a stop signal. If the first AUG triplet is flanked by suboptimal sequences, as in s1 mRNA, some 40 S ribosomes bypass that site and initiate at the next AUG downstream. The second AUG in the s1 message conforms to the consensus sequence (A-N-N-A-U-G-G) for eukaryotic initiation sites.