Scanning of the 50 untranslated region
and AUG recognition
Once assembled near the 50 end of the mRNA, the 43S
complex has to locate the appropriate start codon on the
mRNA. A linear movement along the mRNA 50 UTR in search
of an AUG triplet, termed ‘scanning’, was originally proposed
by Kozak(16,52) (Fig. 1). It explains well the adherence of most
known mRNAs to the ‘first-AUG’ rule, which predicts that
initiation should occur on the start codon closest to the mRNA
50 end. It is also consistent with a large body of genetic and
biochemical data,(52) although it needs to be noted that ‘scanning’
has not been directly demonstrated to date.