Evidence for Involvement of trans-Acting Factors in Selection of the AUG Start Codon during Eukaryotic Translational Initiation

The molecular mechanism with which an appropriate AUG codon is selected as the start site for translational initiation by eukaryotic ribosomes is not known. By using a cell-free translation system, small RNA molecules containing single AUG codons, surrounded by various nucleotide sequences, were tested for their abilities to interfere with the translation of a reporter mRNA. RNAs containing the AUG in an ACCAUGG context (Kozak consensus sequence) were able to inhibit translation of the reporter mRNA. In contrast, RNAs containing the AUG in a less favorable context for start site selection (for example, CAGAUGG) had no effect on the translation of the reporter mRNA. The effect mediated by the ACCAUGC-containing RNAs was not due to sequestration of ribosomal subunits or to particular structural features in these RNAs. To identify potential trans-acting factors that might be preferentially bound by ACCAUGG-containing RNAs, ACCAUGG- and CAGAUGC-containing RNAs with a single 4-thiouridine residue at the AUG were incubated with partially fractionated extracts, and AUG-binding proteins were identified after irradiation of the complexes with UV light and subsequent analysis by gel electrophoresis. The analysis (of such complexes in competition experiments revealed that proteins, approximately 50 and 100 kDa in size, were found to bind directly at the AUG codon embedded in the ACCAUGG motif. One of these proteins has been identified as the La autoantigen. These findings indicate that trans-acting factors may play a role in AUG start site selection during translational initiation.