Abstract
Electrophoretic patterns of phosphoglucose isomerase (PGI) in bony fishes provide strong evidence for a model of genetic control by two independent structural gene loci, most likely resulting from a gene duplication. This model is confirmed by a comparison of certain kinetic and molecular properties of the PGI homodimers (PGI-1 and PGI-2) isolated from extracts of the teleost Astyanax mexicanus. In addition, in most higher teleosts examined, the PGI enzymes show a regular pattern of tissue distribution, with PGI-2 predominant in muscle, the heterodimer often strongest in the heart, and PGI-1 predominant in liver and other organs. An examination of 53 species of bony fishes belonging to 38 families indicates a widespread occurrence of duplicate PGI loci and an early origin of the gene duplication, perhaps in the Leptolepiformes. The apparent presence of three PGI loci in trout and goldfish exemplifies how new loci can be incorporated into the genome through polyploidization.
Highlights
We present the evidence on which this conclusion is based, including an analysis of certain kinetic and molecular properties of the phosphoglucose isomerase (PGI) homodimers in A. mexicanus
On the basis of a study of the tissue distribution and occurrence of multiple PGI loci in a wide variety of bony fishes (Osteichthyes), we propose a hypothesis of the time of origin of the gene duplication and the course of the divergent specialization of the homodimeric and heterodimeric enzyme forms
The patterns of PGI activity on the gels are similar to those for the multiple loci encoding malate dehydrogenase in several fishes (Bailey et al, 1970; Whitt, 1970; Avise and Selander, 1972). Individuals homozygous at both PGI loci show three widely spaced bands. (Other paler bands slightly anodal to the slowest are interpreted as secondary or satellite bands.) Presumably, the three bands result from association of two types of polypeptide subunits, one type encoded by each locus, to produce AA, AB, and BB dimers
Summary
Almost all vertebrates and invertebrates far studied possess only a single phosphoglucose isomerase (PGI) gene locus: m a n ( Detter et al, 1968), deer ( Ramsey et al, 1972), rodents A Department of Chemistry and the Clayton Foundation Biochemical Institute, The University of Texas at Austin, Austin, Texas. Yoshida and Carter (1969) observed multiple electrophoretic bands of phosphoglucose isomerase, representing at least two structurally different isozymes, in rabbit hemolysates and muscle extracts. The genetic basis of three isozymes observed by Nakagawa and Noltmann (1967) in brewers and bakers yeast was not determined
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