EFFECT OF LEADER PRIMARY STRUCTURE ON THE TRANSLATIONAL EFFICIENCY OF PHOSPHOGLYCERATE KINASE MESSENGER-RNA IN YEAST

In order to determine the effect of nucleotide composition of the 5′‐untranslated (leader) region on the translational efficiency of mRNA in yeast, we replaced a large part of the leader region of the phosphoglycerate kinase (PGK) gene by various deoxyoligonucleotides of defined sequence. All mutations left the context of the transcription initiations site and AUG start codon intact. The mutant genes were introduced into yeast cells on a multicopy vector and the ratio of the steady‐state levels of PGKmRNA and protein were determined. We found the translational efficiency to be unaffected by the presence of either an 18 nucleotides (nt) long poly A or poly C tract or by sequences consisting of mixtures of A and C residues in any proportion. In contrast, a polyU tract, as well as mixtures of U and C residues, reduced translational efficiency by a factor of two to three, presumably by long‐rang base ‐pairing between the leader and sequences elsewhere in the coding or 3′‐non‐coding regions of the messenger. In agreement with this hypothesis, a five‐fold reduction in translational efficiency was found for an mRNA carrying a polyC tract in the leader as well as a polyG tract in the trailer, neither of which had any effect on translational efficiency by itself. Therefore, we conclude that the leader and trailer regions (including the polyA tail) of PGK mRNA are sufficiently close to base‐pair when containing complementary sequences. The resulting secondary structure evidently constitutes a barrier for incoming 40S subunits on their way to the AUG start codon. The presence of an 18 nt long polyG tract in the leader completely abolished translation of the PGK mRNA in accordance with earlier observations. However, we found the leaders containing up to 40% G residues interspersed with either A or U, still allow highly efficient translation. This value is about four times as high as the average G content of leader sequence in naturally occurring yeast mRNAS. Finally, neither deletion of about 40% of the trailer sequence of PGK mRNA, not replacement of this sequence by homopolymer tracts had any effect on translational efficiency. Copyright © 1990 John Wiley & Sons Ltd.