The byp1-3 allele of the Saccharomyces cerevisiae GGS1/TPS1 gene and its multi-copy suppressor tRNAGLN (CAG): Ggs1/Tps1 protein levels restraining growth on fermentable sugars and trehalose accumulation

Byp1-3 is an amber nonsense allele of the Sacchromyces cerevisiae GGS1/TPS1 gene which encodes the small subunit of the trehalose synthase complex. Mutations in this gene confer an inability to grow on glucose or fructose but the phenotype of byp1-3 mutants is leaky in a strain-dependent manner. Overexpression of the isolated byp1-3 allele suppressed the growth defect of a ggs1/tps1 mutant. Expression of an in-vitro-generated mutant allele of GGS1/TPS1 that lacks all the coding sequences downstream from the byp1-3 mutation led to the production of a shortened protein that did not complement the ggs1/tps1 mutant. We have isolated, as an allele-specific multi-copy suppressor of the growth defect of the byp1-3 mutant on fructose, the gene for tRNA^GLN (CAG). Thus the leaky phenotype of byp1-3 mutants is due to a low level of read through of the internal nonsense codon by tRNA^GLN (CAG). Using overexpression of the isolated byp1-3 allele, as well as of the tRNA^GLN (CAG) gene, we were able to demonstrate that as little as about 10% of the normal Ggs1/Tps1 protein level is sufficient for slow growth on fructose. We also show a correlation between the level of Ggs1/Tps1, the ability to accumulate trehalose in stationary phase and the ability to grow on fermentable sugars. Sequence analysis of the cloned tRNA^GLN (CAG) gene showed that it is located 700 bp upstream of URA10. However, we found considerable differences to the reported sequence of URA10, in particular in the non-coding region.Communicated by K. Wolf