A third independently segregating mitochondrial MDH structural gene has been localized to 3L

Goodman et al. (MNL 52:99) identified variants of a third mitochondrial MDH locus, MdhC, which is distinct from the other two mitochondrial genes, A and B (the 6L gene). Unfortunately, on a tris-citrate, pH 7.0 starch gel system, the homodimer of the most common allele of the C gene (C-16) migrates to a position obscured by the broad and intensely-staining s-1 band, composed largely of the soluble MDH's.

In our lab several methods have been used to demonstrate the fact that there is a residual activity at the s-1 position which is not due to the sMDH's: e.g. (1) Genetic removal of the sMDH's from the normal (N) s-1 position as in the homozygous "null slow" pattern of Fig. 2-2. Under these conditions, there is a faint band discernible at the N position. This band does not form a heterodimer with the sMDH slow variant. (2) Goodman, et al. have shown that use of an ascorbate-sucrose homogenizing medium selectively eliminates sMDH's from the starch gel zymograms. Although this technique causes a slight smearing and greater separation of the mitochondrial isozymes, they are still visible and scoreable. Both Goodman and L. Bauman kindly provided kernels of the H25 line--which is homozygous for a variant (C-18) migrating to a position well above the obscuring s-1 N band. In outcrosses of this line a heterodimer forms between the C-18 and s-1 "normal" position. Elimination of the sMDH's with sucrose-ascorbate reveals an approximate 1:2:1 staining intensity ratio of the C-18 homodimer:C-18.C-16 heterodimer:C-16 homodimer.

The H-25 line was crossed with the B-A translocation lines, per previously described protocol. The following TB's uncover the C-18 variant: TB-3Ld, TB-3Lc and TB-1La-3L5267. TB-1La does not uncover the C-18 variant. Dr. J. Beckett generously provided the TB-3Lc and 3Ld stocks which were then crossed to Birchler's a-m-1 R-scm tester. Therefore genetically defined hyperploid plants were used for the crosses with C-18. Approximately one-third of the kernels tested from an H25 x TB-3Ld cross uncovered C-18. (Although in a hemizygote, the homodimer band was difficult to see, such kernels were easy to identify by a lack of the appropriate heterodimers.) The fact that the compound TB-1La-3L5267, whose 3L breakpoint is listed at 0.73, uncovers the C-18 variant, indicates that the MdhC gene is located in the distal region of the long arm of chromosome 3. Genetic linkage studies are in progress.

Kathleen J. Newton

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