UGA suppressors in Saccharomyces cerevisiae: allelism, action spectra and map positions

Sixty independent UGA suppressors of Saccharomyces cerevisiae have been studied. They are dominant and are divided into 16 groups (loci) by recombination. Suppressors representing these loci are divided into two classes by action spectra; four in class 1 (a broad action spectrum) and 12 in class 2 (a narrow action spectrum). Class 1 suppressors are less frequent in terms of not only total number but also number per locus than class 2 suppressors, indicating difference in either or both mutation frequency and selective pressure between suppressors of the two classes. Two of the class 1 suppressors, SUP152 and SUP161, do not recombine with SUP28 and SUP33, leucine-inserting UAA suppressors, respectively, indicating that they are mutations in genes coding for tRNA(Leu)UUA. Of the remaining two class 1 suppressors, SUP160 which causes lethality in the psi+ cytoplasm is mapped on chromosome XV very close to the centromere, and SUP165 on the right arm of chromosome XIV 44 cM distal to lys9. Of the class 2 suppressors, ten do not recombine with one or another of previously known UGA suppressors. The remaining two class 2 suppressors, SUP154 and SUP155, are mapped on the left and right arms of chromosome VII, respectively.