SERINE-INSERTING UAA SUPPRESSION MEDIATED BY YEAST TRNASER

[摘要] The number of loci that give rise to serine-inserting UAA suppressors in the yeast S. cerevisiae was determined by examining > 100 of the revertants that suppressed the 2 UAA markers his4-1176 and leu2-1; the his4-1176 marker is suppressed by serine-inserting but not by tyrosine- or leucine-inserting suppressors and the leu2-1 marker is suppressed by all UAA suppressors. The suppressors could be assigned to 1 or other of the 4 loci: SUP16 and SUP17, which were previously known to yield serine-inserting suppressors, and SUP19 and SUP22. The chromosomal map position of SUP19 suggested that it may be allelic to the previously reported suppressor SUP20, while the SUP22 suppressor has not been described. Representatives of all the 4 suppressors insert serine at the UAA site in iso-1-cytochrome c from suppressed cyc1-72 strains. The degree of suppression by the serine-inserting suppressors was SUP16 > SUP17 > SUP19 > SUP22. The efficiency of suppression of each of the 4 serine suppressors was increased by the chromosomal mutation sal and by the cytoplasmic determinant .psi.+. Read-through of the synthetase gene of the RNA bacteriophage Q.beta. in a cell-free system was used to demonstrate that tRNASer from SUP16, SUP17 and SUP19 strains can translate UAA codons. In contrast, tRNASer or total tRNA from SUP22 strains had no suppressing activity. The 3 loci SUP16, SUP17 and SUP19 encode iso-accepting species of tRNASer, and that the UAA suppression is mediated by mutationally altered tRNA molecules. The mechanism of SUP22 suppression remains unknown.