Gene-code variety Essay

In this heyday of genetic engineering, scientists take great advantage of the uniformity of the DNA code. A human cell’s gene, for example, once put in the proper context, can be effectively translated by the decoding equipment of a bacterium. But the genes of one group of organisms, single-celled animals called ciliates, are not translated well by the cellular machinery of other organisms. Now scientists report “the surprising finding” that the genetic coding of these simple animals actually departs from the so-called “universal” code. The only coding variations previously discovered are in genes of subcellular structures, the mitochondria (SNd9/15/79, p.185).

Stuart Horowitz and Martin A. Gorovsky of the University of Rochester in New York determined the nucleotide (DNA subunit) sequence of two genes in the nuclei of the ciliate. Tetrahymena thermophila. In the standard code, the sequence TAA is a “stop” signal, terminating the translation of a gene. But in these Tetrahymena genes, TAA corresponds to the amino acid glutamine.

Two groups of scientists report in the March 14 NATURE analyses of genes of another ciliate, Paramecium. Francois Caron and Eric Meyer of the National Center of Scientific Research in Gif-sur-Yvette, France, find that both TAA and TAG, which normally terminate gene translation, are scattered through the genes. Analysis of a specific Paramecium gene by J. R. Preer Jr. and colleagues at Indiana University in Bloomington indicates that TAA and TAG code for glutamine. In addition, work in West Germany indicates that TAA encodes glutamine in several genes in another ciliate, Stylonychia. In all these ciliates, only one of the usual stop codons, UGA, appears to act as a termination signal.

Under most conditions, any genetic change involving termination signals is likely to be lethal to an organism. A major puzzle now is how the variation in the genetic code of ciliates could have arisen during evolution.

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