Module Organization in Proteins and Exon Shuffling
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Molecular mechanisms leading to drastic evolutionary change is essentially a recombination of genetic information. Exon shuffling, is one such mechanism used in the creation of novel proteins. The remnants of the shuffling are observed in the split gene structures of eukaryotic cells. Since it was found that the introns in the genes of the globin family corresponded to module joints of the globin chains, such correspondence has been widely observed in various genes. Modules are defined as compact conformational units in the three-dimensional structures of globular proteins. This article focuses on the close correlation between intron positions and module boundaries in several genes and their products. The intron-module relationship shows that exon shuffling is module shuffling in protein language. However, no introns are found at some module joints. It is suggested that many introns were lost during evolution. Though most of the eukaryotic genes are split by introns, introns are located in only a few exceptional genes in the case of prokaryotes. Module organization in proteins is observed also in prokaryotes as well as in eukaryotes. It is highly possible that prokaryotic genes were split by introns and these introns were lost after prokaryote-eukaryote divergence. Imperfect conservation of introns in eukaryotes shows that the module organization, conserved in the three-dimensional structures of contemporary proteins, gives us useful information concerning the evolutionary history of exon shuffling
KeywordsModule Organization Globin Gene Globular Domain Module Boundary Triosephosphate Isomerase
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