Divergent and concerted evolution of the two regions encompassing the iso-1- cytochrome c and iso-2-cytochrome c genes of yeast : (saccharomyces cerevisiae, cytochrome c of yeast, evolution of yeast, cloning yeast genes, recombinant DNA, transpositions, composite genes)
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The yeast Saccharomyces cerevisiae contains a cluster of three genes CYC1-0SM1-RAD7, denoted COR, located on the right arm of chromosome X and a cluster of three genes ANP1-RAD23 -C YC7, denoted ARC, located on the left arm of chromosome V. Both COR and ARC are in approximately 6 kb long regions and there are striking similarities between CYCl and CYC7, between OSMl and ANP1 and between RAD7 and RAD23. The relationships between COR and ARC suggest that one of the regions arose during evolution by a transposition in which a circular intermediate containing the three genes integrated at a new chromosomal position. The order of the genes could have been altered if the circle closed and opened at different sites during the excision and integration, respectively. Subsequently, the primordial COR and ARC genes could have acquired slightly different properties or functions by divergent evolution. The hypothetical transposit ion that occurred during evolution superficially resembles transpositions observed in certain COR2 strains in which segments containing the entire COR region are transposed to various positions on different chromosomes. In addition to the evolutionary development by means of transpositions and divergencies, identical short stretches observed within the CYCl and CYC7 genes, as well as in other duplicated genes from yeast and other organisms, suggest the occurrence of concerted evolution. The possibility of concerted evolution has been directly demonstrated with the formation of the so-called composite genes, in which the central portions of the CYCl locus is replaced with a corresponding segment from the CYC7 locus by gene conversion. Thus, by considering the structures of the COR and ARC regions, we suggest that these two regions developed by three evolutionary processes: a transposition, divergent evolution and concerted evolution. The transposition of COR regions and the formation of CYC1-CYC7 composite genes superficially resemble two of the evolutionary steps.