Inhibition of Growth by Amber Suppressors in Yeast

Abstract

Strains of the yeast Saccharomyces cerevisiae that contain highly efficient amber (UAG) suppressors grow poorly on nutrient medium, while normal or nearly normal growth rates are observed when these strains lose the suppressors or when the suppressors are mutated to lower efficiencies. The different growth rates account for the accumulation of mutants with lowered efficiencies in cultures of strains with highly efficient amber suppressors. Genetic analyses indicate that one of the mutations with a lowered efficiency of suppression is caused by an intragenic mutation of the amber suppressor. The inhibition of growth caused by excessive suppression is expected to be exacerbated when appropriate suppressors are combined together in haploid cells if two suppressors act with a greater efficiency than a single suppressor. Such retardation of growth is observed with combinations of two UAA (ochre) suppressors (Gilmore 1967) and with combinations of two UAG suppressors when the efficiencies of each of the suppressors are within a critical range. In contrast, combinations of a UAA suppressor and a UAG suppressor do not affect growth rate. Apparently while either excessive UAA or excessive UAG suppression is deleterious to yeast, a moderate level of simultaneous UAA and UAG suppression is not.

Isolation and Characterization of Amber Suppressors in Yeast

“…In these pedigrees, suppression was EO longer associated with poor growth. These observations, reported in detail in the accompanying paper (LIEBMAN and SHERMAN 1976), suggested that the efficient amber suppressors could become low-efficiency suppressors, and that poor growth was caused by the efficient amber-suppression.…”

“…Some of these classes may not represent differences that are necessarily attributable to a single suppressor, since genetic analysis of many suppressors was not undertaken and it is unknown whether or not the same phenotype would be observed in the sexual progeny after crossing. Evidence described in the accompanying paper ( LIEBMAN and SHERMAN 1976) and elsewhere shows that the pattern of suppression, or the class number in Table 3, of certain suppressors can be modified by other mutations. Furthermore, the apparent suppression of cycl-179 was found in a few strains to be due to increased levels of iso-2-cytochrome c, rather than to suppression of the UAG triplet in the mRNA of iso-l-cytochrome c. Also, falsenegative scores in Table 3 were obtained with strains unable to grow on complete glycerol medium and which grew poorly on complete glucose medium.…”

“…F3r example, the UAG suppressors obtained in this study by mutations of the UAA suppressors SUPS-01, SUP7-01 and SUQ5-01 are denoted, respectively as SUPS-01-ai, SUP7-01-a1 and SUQS-01-ai, or briefly as SUPS-a, SUP7-a and SUQ5-a. The symbol SUPS-a2-al denotes a suppressor arising by a secondary mutation of SUP5-aZ that lowered the efficiency of suppression and that is described in the accompanying paper (LIEBMAN and SHERMAN 1976).…”

“…In some cases low-temperature spectroscopy of the fermentor-grown cells indicated that they contained less than 15% of the wild-type level of cytochrome c even though greater than 50% of the wild-type level of iso-1cytochrome c was present in the original inoculum of the suppressed cycl-I79 mutant. Such batches were either discarded, or were pooled and analyzed separately from batches containing a high level of cytochrome c. This phenomenon, which reflects an instability of the suppressors, is described in detail in the accompanying paper (LIEBMAN and SHERMAN 1976).…”

“…Thus we believe that the random spore tests accurately identified the loci of the amber suppressors. Suppressors acting efficiently on cycl-179 can mutate so that there is inefficient or no suppression of the cycl-179 marker while there is still suppression of at least some of the nutritional markers (LIEBMAN and SHERMAN 1976). The inefficient suppression was shown to be due to the result of mutations either of the suppressor gene or of extraneous genes.…”