Effect of deletions in the 5'-noncoding region on the translational efficiency of phosphoglycerate kinase mRNA in yeast.

Deletions of various sizes were introduced into the region of the yeast PGK gene encoding the 5'-nontranslated portion of the phosphoglycerate kinase (PGK) mRNA. The effect of these deletions on the translational efficiency of the mutant transcripts was analysed by assaying the levels of mutant PGK mRNA and PGK protein in cells transformed with the mutant genes. Quantification of transcript levels by either Northern analysis or a reverse transcription assay demonstrated that there were no significant differences in the levels of mutant PGK mRNA between the various mutants. Thus, the leader sequence does not appear to play a role in determining the relatively long half-life of yeast PGK mRNA. Analysis of PGK protein levels in the various mutants revealed no effect when the length of the leader was reduced from 45 to 27 nucleotides (nt). Protein levels dropped by about a factor 2, however, upon a further decrease to 21 nt. Additional shortening did not cause a further dramatic reduction in translational yield. Even an mRNA containing a leader of only 7 nt was still translated at about 50% of the optimal rate. Therefore, while optimal translation of a yeast mRNA requires a leader length of at least some 30 nt, shorter leaders still allow considerable translation to take place.