Mutants of yeast initiating translation of iso-1-cytochrome c within a region spanning 37 nucleotides.

We used a specially constructed strain, cyc1-345, of the yeast Saccharomyces cerevisiae to isolate revertants that initiated translation of iso-1-cytochrome c at various sites along an extended region of the mRNA. Normal amounts of iso-1-cytochrome c occurred when translation initiated at the abnormal sites corresponding to amino acid positions -3, -2, 3 and 5, as well as the normal position -1; 20% of the normal amounts occurred when translation initiated at the abnormal position 9. These results with cyc1-345 revertants indicate that translation of iso-1-cytochrome c can initiate with the normal efficiency at any site within the region spanning 25 nucleotides. Furthermore, because the lower amount of the short iso-1-cytochrome c in the mutant initiating at position 9 may not necessarily reflect an inefficiency of translation, we believe that translation can initiate with normal or near-normal efficiencies at any site within a 37 nucleotide region, and presumably at any site preceding and following that of the normal initiation codon. These results establish that there is no absolute requirement for a particular sequence 5' to the initiation codon, and are consistent with our previous suggestion that translation starts at the AUG codon closest to the 5' end of the mRNA.