Independent spontaneous mitochondrial malate dehydrogenase null mutants insoybean are the result of deletions

The mitochondrial malate dehydrogenase-1 (Mdh1) gene of soybean [Glycine ma x (L.) Merr.] spontaneously mutates to a null phenotype at a relatively hig h rate. To determine the molecular basis for the instability of the Mdhl ge ne, the gene was cloned and sequenced. The null phenotype correlated with t he deletion of specific genomic restriction fragments that encode the Mdhl gene. The composition of the Mdhl gene and its environs were compared with those of the more stable MDH2 gene. Several possible causes of the observed instability were found, including duplications, repeats, and two regions w ith similarity to a soybean catalase. The most likely cause of instability, however, appeared to be a 1233 bp region with 58.9% identity to the Cyclop s retrotransposons. Translation of a 714 bp segment of this region produced a peptide composed of 238 amino acid residues that showed 35-40% identity and 55-60% similarity to several putative Cyclops gag-pol proteins (group-s pecific antigen polyprotein). This short peptide also contained a segment t hat corresponded to the protease active site of the gag-pol protein. Thus i n an appropriate genetic background, a retrotransposon, whether whole or fr actured, could promote genetic rearrangements.