A New Genetic Code in Yeasts

The ascomycete yeasts, which include the model organism Saccharomyces cerevisae, are a diverse group of fungi spanning a billion years of evolution, and with enormous biotechnological potential. Unique among eukaryotes, some yeasts are known to have a genetic code change, in which CUG codons are translated to serine rather than the usual leucine. Such genetic code changes are interesting due to their extreme rarity, but may also complicate efforts to exploit the yeasts’ biotechnological potential through exogenous expression of genes (which might have genetic codes incompatible with hosts). We sequenced the genomes of 16 ascomycete yeasts with the goal of discovering biotechnologically useful genes. From the genome sequences, we predicted that a xylose-fermenting yeast, Pachysolen tannophilus, translates CUG codons neither to canonical leucine nor previously known serine, but to alanine. Mass-spectrometry proteome analysis and genetic transformation experiments confirmed this prediction. The existence of multiple genetic codes in the ascomycete yeasts has implications for their exploitation in biotechnology and synthetic biology.

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