Cryptic genes and genetic codes in mitochondria
Friday seminar by Hervé Seligmann
DNA replication limits cell division rate, hence selection tends to decrease genome size, especially for endosymbionts such as mitochondria. Overlap coding in frameshifted gene sequences increases coding information without lengthening genomes, but stop codons in frameshifted sequences block decoding.Expression of tRNAs decoding stop codons induces translation of cryptic protein coding genes, according to a stopless genetic code detected by alignment analyses. In mitochondria, AGR stops code for arginine in primates, for lysine in turtles at large and glycine in the marine Olive Ridley turtle, Lepidochelys olivacea. UAR stops code for tryptophan in turtles and serine in Drosophila. About 50 percent of all regular mitochondrial genome codons are involved in overlap coding. Mitochondrial genomes code for two parallel genetic systems, by the regular code and the stopless code. In Lepidochelys, the hierarchy between the codes is switched: many regular mitochondrial proteins usually coded by the regular mitochondrial genetic code are coded by the stopless code, and vice versa, perhaps due to its marine habits. The natural history of these parallel codes remains to be explored, especially for non-mitochondrial genomes.
Department of Ecology, Evolution and Behaviour, The Hebrew University of Jerusalem, Jerusalem 91404 Israel; Department of Life Sciences, Ben Gurion University, 84105 Beer Sheva, Israel.