A "null activity" allele of the mitochondrial MDH gene on chromosome six

In a Knobless Wilbur line obtained from the Maize Genetics Coop, we discovered an odd variant of the mMDH pattern. This line contained the m-3 homodimer and the m-4 heterodimer bands but lacked an m-5 homodimer. F2 data from outcrosses (Fig. 5-1) of this line showed that the Knobless Wilbur variant segregated as an allele of the gene we have localized to chromosome six. The fact that we were seeing a heterodimer (m-4) band suggested to us that the locus on 6L was producing a mutant protein which is capable of forming heterodimers with active subunits but which is itself catalytically inactive. Such null activity CRM+ alleles are known to exist for the Adh1 locus (D. Schwartz and T. Endo, Genetics, 1966).

We tested this hypothesis genetically with the cross depicted in Fig. 5-2. The variant pattern (Fig. 5-2A) in this cross is homozygous. Both alleles of the 6L locus encode products which homodimerize at the m-3 position. The second gene (MdhA, whose location is not yet confirmed) is homozygous for an allele encoding a slow migrating mitochondrial variant--the m-7 band. The A.B heterodimer in this case migrates to a position between m-4 and m-5. A cross between this line and Knobless Wilbur results in an F1 pattern which includes the hybrid band, m-6, that is normally formed when the m-5 and m-7 bands are present. These results support the interpretation that the Knobless Wilbur line contains a B gene activity null which is capable of forming heterodimers with two different A gene variants. Crosses of the Knobless Wilbur activity null by TB-6Lc demonstrated that this variant was also uncovered.

Figure 5-1.

Figure 5-2.

Kathleen J. Newton


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