GENETIC CONTROL OF MALATE DEHYDROGENASE ISOZYMES IN MAIZE

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Abstract
At least six nuclear loci are responsible for the genetic control of malate dehydrogznase (L-malate: NAD oxidoreductase; EC 1.1 .I .37; MDH) in coleoptiles of maize. Three independently segregating loci (Mdh1, Mdh2, Mdh3) govern the production of MDH isozymes resistant to inactivation by ascorbic acid and found largely or solely in the mitochondria. A rare recessive allele found at a fourth nuclear locus (mmm) causes increased electrophoretic mobility of the MDH isozymes governed by the Mdh1, Mdh2 and Mdh3 loci. -Two loci (Mdh4, Mdh5) govern MDH isozymes that are selectively in- activated by homogenization in an ascorbic acid solution and that appear to be nonmitochondrial (soluble). Mdh4 and Mdh5 segregate independently of each other and independently of Mdhl, Mdh2 and Mdh3. However, there is close linkage between the migration modifier and Mdh4.—Multiple alleles have been found for all of the Mdh loci except the migration modifier, and electrophoretically "null" or near "null" alleles (as expressed in standardized sections of maize coleoptile) have been found for all loci except Mdh4. Duplicate inheritance commonly occurs for Mdhl and Mdh2 and also for Mdh4 and Mdh5.—Inter- and intragenic heterodimers are formed between sub-units specified by the three loci governing the mitochondrial MDH isozymes. The same is true of the alleles and nonalleles at the two loci governing the soluble variants. No such heterodimers are formed by interactions between mitochondrial and soluble MDH isozymes.