An mRNA sequence derived from a programmed frameshifting signal decreases codon discrimination during translation initiation

Abstract

Sequences and structures in the mRNA can alter the accuracy of translation. In some cases, mRNA secondary structures like hairpin loops or pseudoknots can cause frequent errors of translational reading frame (programmed frameshifting) or misreading of termination codons as sense (nonsense readthrough). In other cases, the primary mRNA sequence stimulates the error probably by interacting with an element of the ribosome to interfere with error correction. One such primary mRNA sequence, the Ty3 stimulator, increases programmed +1 frameshifting 7.5 times in the yeast Saccharomyces cerevisiae. Here we show that this stimulator also increases the usage of non-AUG initiation codons in the bacterium Escherichia coli but not in S. cerevisiae. These data suggest that in E. coli, though not in yeast, an element of the ribosome's elongation accuracy mechanism ensures initiation accuracy.

FIGURE 1.

Lack of effect of varying spacing between the Ty3 stimulator and the initiation codon. The figure shows expression of β-galactosidase from plasmids with increasing spacing between the Ty3 stimulator and the initiation codon (see Table 2 for sequences of the initiation regions). For each construct, the observed units of β-galactosidase (nanomoles of orthonitrophenyl-β-galactopyranoside cleaved per minute per OD₆₀₀ of culture) are shown; constructs using an AUG (dashed line) or UUG (dotted line) initiation codon. The expression of the UUG constructs is shown as a percentage of the expression of the corresponding AUG construct (solid line).