Ascoidea asiatica stochastically encodes CUG as leucine and serine
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It is the only known example of a proteome with non-deterministic features
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Stochastic encoding is caused by competing tRNALeu(CAG) and tRNASer(CAG)
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A. asiatica copes with stochastic encoding by avoiding CUG at key positions
Summary
Although the “universal” genetic code is now known not to be universal, and stop codons can have multiple meanings, one regularity remains, namely that for a given sense codon there is a unique translation. Examining CUG usage in yeasts that have transferred CUG away from leucine, we here report the first example of dual coding: Ascoidea asiatica stochastically encodes CUG as both serine and leucine in approximately equal proportions. This is deleterious, as evidenced by CUG codons being rare, never at conserved serine or leucine residues, and predominantly in lowly expressed genes. Related yeasts solve the problem by loss of function of one of the two tRNAs. This dual coding is consistent with the tRNA-loss-driven codon reassignment hypothesis, and provides a unique example of a proteome that cannot be deterministically predicted.