[Structure and the mode of expression of isozyme genes]

Isozymes are the different molecular forms of an enzyme that catalyze the same reaction. They have been known to show the different distribution and metabolic roles within one type of cell or in different types of cells. Three evolutionary strategies led to generate isozymic forms; (1) multiple forms encoded by separate genes, (2) those generated by allelic mutations, and (3) those generated from a single gene by either alternative usage of the protein-coding exons or alternative usage of multiple translation initiation sites. In general, genes encoding different types of isozyme show a similarity in the number and length of the protein-coding exons but not in the 5'- and 3'-non-coding exons and also in the introns. The expression of isozyme genes is regulated temporally or constitutively through the interaction of the cis-acting elements on regulatory regions of the genes with basal- and tissue-specific transcription factors.