5′-Noncoding sequences have been tabulated for 211 messenger R'JAs from higher eukaryotic cells. The 5′-proximal AUG triplet serves as the initiator codon in 95% of the mRNAs examined. The most conspicuous conserved feature is the presence of a purine (most often A) three nucleotides upstream from the AUG initiator codon; only 6 of the mRNAs in the survey have a pyrimidine in that position. There is a predominance of C in positions ?1, ?2, ?4 and ?5, just upstream from the initiator codon. The sequence (G) thus emerges as a consensus sequence for eukaryotic initiation sites. The extent to which the ribosome binding site in a given mRNA matches the ?1 to ?5 consensus sequence varies: more than half of the mRNAs in the tabulation have 3 or 4 nucleotides in common with the CCACC consensus, but only ten mRNAs conform perfectly.
展开▼
