Functional importance of RNA interactions in selection of translation initiation codons.

RNA base pairing between the initiation codon and anticodon loop of initiator tRNA is essential but not sufficient for the selection of the 'correct' mRNA translational start site by ribosomes. In prokaryotes, additional RNA interactions between small ribosomal subunit RNA and mRNA sequences just upstream of the start codon can efficiently direct the ribosome to the initiation site. Although there is presently no proof for a similar important ribosomal RNA interaction in eukaryotes, the 5' non-coding regions of their mRNAs and 'consensus sequences' surrounding initiation codons have been shown to be strong determinants for initiation-site selection, but the exact mechanisms are not yet understood. Intramolecular base pairing in mRNA and participation of translation initiation factors can strongly influence the formation of mRNA-small ribosomal subunit-initiator tRNA complexes and modulate translational activities in both prokaryotes and eukaryotes. Only recently has it been appreciated that alternative mechanisms may also contribute to the selection of initiation codons in all organisms. Although direct proof is currently lacking, there is accumulating evidence that additional cis-acting mRNA elements and trans-acting proteins may form specific 'bridging' interactions with ribosomes during translation initiation.