Influences on translation initiation and early elongation by the messenger RNA region flanking the initiation codon at the 3' side.

The downstream region (DR) located immediately after the initiation codon acts as a translational enhancer and depending on its sequence gene expression can vary considerably. In order to determine the influence of the DR on the apparent translation initiation, we have analyzed several naturally occurring DRs (a stretch of five codons) in a lacZ reporter gene. The efficiency of expression, associated with these DRs did not show any correlation to the expression levels connected with the natural genes. Changes of the iso-codon composition in the DR, thus maintaining the amino acid sequence in the gene product, gave significant variations in gene expression. Thus, the messenger RNA base sequence, and not the encoded amino acid sequence, in the early coding region is the determinant for the apparent efficiency of translation initiation and/or early elongation.