Deviations from the Universal Code

By the end of the 1970s and during the 1980s it was discovered that the universality of the genetic code is not absolute, and some exceptions are possible (Barrell et al., 1979; Yamao et al., 1985). Among living organisms, now two genera of eubacteria, Mycoplasma and Spiroplasma, are known to have two codons for tryptophan, the universal UGG and the "neighboring" UGA, which is a stop codon in other organisms. In one genus of Ciliates (Protozoa), Euplotes, UGA codes for cysteine. Two other universal stop codons, UAA and UAG, were reported to code for glutamine in other genera of Ciliates (Tetrahymena, Paramecium, Stylonicia, Oxytricha) and in at least one genus of unicellular green algae (Acetabularia). Also, in some yeast (Candida) the universal leucine codon CUG codes for serine. The known cases of variations in the genetic code are summarized in Table 2.1 (see Watanabe & Osawa, 1995). Further exceptions of the universal genetic code may be discovered in future, especially in unicellular Eukaryotes (Protozoa, algae and fungi).

Organelles of eukaryotic cells, including mitochondria, possess their own protein-synthesizing systems. The protein-synthesizing systems of animal and fungal (but not plant) mitochondria typically show a number of significant deviations from the universal code (Table 2.2). Tryptophan in these mitochondria is coded by both UGG and UGA; UGA is therefore not used as a termination codon. In mitochondria of all Vertebrates, most (but not all) Invertebrates and some fungi the universal isoleucine codon AUA codes for methionine, so that methionine is determined there by two triplets, the universal AUG and the "neighboring" AUA. The triplets AGA and AGG do not code for arginine in mitochondria of most animals; they are stop codons in vertebrate mitochondria and codons for serine in mitochondria of many Invertebrates (Echinoderms, insects, mollusks, nematodes, Platyhelminthes). In yeast mitochondria (Saccharomyces, Torulopsis) the whole codon family CUU, CUC, CUA, and CUG codes for threonine but not for leucine, although in other fungi, such as Neurospora and Aspergillus, these codons correspond to leucine as given by the universal code.

Table 2.1. Variations in eubacterial and in nuclear genetic code from "universal" genetic code.


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