BGN 2: Chromosomal location of nuclear genes coding for mitochondrial malate dehydrogenase BARLEY GENETICS NEWSLETTER, VOL. 2, II. RESEARCH NOTES
McDaniel, pp. 49-51

II.17. Chromosomal location of nuclear genes coding for mitochondrial malate dehydrogenase.

R. G. McDaniel. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, U.S.A.

Evidence for nuclear gene control of a mitochondrial enzyme, malate dehydrogenase, has been presented (Longo and Scandalios, Proc. Natl. Acad. Sci. U.S. 62:104, 1969). These data indicated that genetic variants of mitochondrial malate dehydrogenase (m-MDH) were inherited according to classical Mendelian concepts. Numerous workers have used 3N endosperm to produce genetic ratios for inheritance of enzyme variants in order to ascertain their control by nuclear genes. Dose compensation for enzyme activities has been reported (Seecof et al., Proc. Natl. Acad. Sci. U. S. 62:528, 1969). Nielsen and Frydenberg (Hereditas 67:152, 1971) have applied the concept of gene localization using trisomics to the study of inheritance of esterase enzyme variants. A nullisomic-tetrasomic series in wheat has been used in studying chromosomal location of alcohol dehydrogenase genes (Hart, Genetics Abstracts 61:S-29, 1970). The study reported here is a progress report of my efforts (1) to confirm nuclear genetic control of m-MDH in barley and (2) to establish the chromosomal location of these genes using trisomics.

One major use of plant and animal trisomics is for assigning genes to linkage groups. Location of genes coding for specific proteins on respective chromosomes has been reported (McDaniel and Ramage, Can. J. Genet. Cytol. 12:490, 1970). A logical extension of these techniques would be in establishing the location of genes specifying proteins with enzymatic activity. Presence of an extra chromosome could be hypothesized to result in additional enzyme activity if the gene(s) coding for that enzyme were located on the chromosome in question. Data representative of a series of experiments with these objectives are presented in Table 1.

Table l. Mitochondrial malate dehydrogenase activity of extracts of single scutella from diploids and primary trisomics in Hordeum vulgare L. cv. 'Betzes'. Relative absorbance (x 103) determined spectrophotometrically using a recording densitometer. m-MDH was assayed cytochemically. Gels were incubated with stain for exactly 6 min. at 37°C. Methods follow Grimwood and McDaniel (Biochim. Biophys. Acta 220:410, 1970). Data are averages of 2 experiments in which 2 to 6 trisomics for each chromosome were assayed in a segregating population of 16 seeds for each trisomic line.

These data indicated that presence of an extra chromosome 2, 4, 6 or 7 in nuclei elicited additional enzyme activity in mitochondria from these cells. The most simple case one could hypothesize for gene action would result in 50% additional m-MDH activity in the mitochondria of cells whose nuclei contained the extra chromosome. The extra chromosome would be expected to contribute an additional dose of the gene(s) coding for the component polypeptides of this enzyme. Data of Table 1, in which increases of enzyme activity result from presence of any one of four different chromosomes, suggest more complex control of this enzyme. Recent work has shown that variants of m-MDH are present in populations of barley mitochondria (Grimwood and McDaniel, Biochim. Biophys. Acta 220:410, 1970). The isoenzymes are likely conformers and reflect the tetrameric nature of this enzyme (Kitto et al., Biochim. Biophys. Res. Commun. 38:31, 1970; Shows and Ruddle, Science 160:1356, 1968). There is evidence, however, that these conformers differ in physical properties (above and Grimwood and McDaniel, unpublished data).

The level of enhancement of enzyme activity I observed in the presence of extra chromosomes (116-130% of control) does not approach what one might expect (150%) in an idealized case. Similar problems have been pointed out in studies of human aneuploidy, where difficulty in establishing chromosomal location of genes for serum enzymes in Trisomy 21 has resulted from analytical variability and possible gene regulatory action (King et al. Lancet 2:1302, 1962; Cox, Ann. N. Y. Acad. Sci. 166:406, 1969). Hence, regulatory mechanisms may restrict the enzyme activity resulting in a total only slightly in excess of diploids. Presence of multiple regulatory genes on the same or other chromosomes would further complicate analysis. This would be a distinct possibility in a diploid organism such as barley; where evidence of gene duplication between chromosomes exists. Thus, m-MDH isoenzymes, unlike enzymes without regulatory properties, will require precise biochemical analysis (Munkres, Biochim. Biophys. Acta 220:149, 1970; Benveniste and Munkres, Biochim. Biophys. Acta 220:161, 1970) in order to ascertain the mechanisms of genetic transcription and regulation and their chromosomal basis.

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