In most plant species investigated, phosphohexose isomerase (PHI, or PGI, or GPI) isozymes are dimers and are usually encoded by two loci, with the products of one locus active in the cytoplasm and the other in the plastids. Until recently, in our studies of PHI isozymes in maize, we have dealt only with those found in the cytoplasm, which are encoded by the Phi1 locus. This locus is localized on chromosome 1 between Adh1 (12% recombination) and Gdh1 (5% recombination). Enzymes encoded by this locus have been found in tissues throughout the plant, including roots, leaves, coleoptiles, scutella, and pollen. We have identified nine alleles at this locus.
Recently we have found a second set of PHI isozyme bands, presumably specified by a second locus, Phi2. Isozymes encoded by this locus are found in plastids (etioplasts) and may also be expressed in the cytoplasm. Genetic studies of this locus have not been completed.
C. W. Stuber and M. M. Goodman
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