A cloned cDNA for duck malic enzyme detects abnormally large malic enzyme mRNAs in a strain of mice (Mod-1n) that does not express malic enzyme protein

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Abstract
Sensitive immunochemical assays were used to measure the mass and rate of synthesis of malic enzyme protein in wild-type and Mod-1n mutant mice fed a high carbohydrate/low fat diet supplemented with thyroid hormone. Malic enzyme activity in the fed, wild-type mice was 100-fold higher than in starved, wild-type mice. Neither activity, mass, nor synthesis of malic enzyme could be detected in fed, mutant mice. Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase responded to these dietary manipulations with normal or supranormal increases in activities, respectively, in mutant mice. A c[complementary]DNA clone containing an almost complete copy of the mRNA for malic enzyme from duck liver was used to analyze poly(A+) RNA from C57BL/6J-DBA/2J hybrid mice that were fasted and refed a high carbohydrate/low fat diet supplemented with thyroid hormone. The 32P-cDNA probe hybridized to 2 RNA of 2250 and 2950 nucleotides. The same 2 RNA were detected in RNA from starved mice except at much lower concentrations. A similar analysis of RNA from Mod-1n mice fed the high carbohydrate-thyroid diet also revealed 2 hybridizing RNA but each was 700-800 nucleotides longer than its counterpart in wild-type mice. The abundance of malic enzyme mRNA in the fed, mutant mice was about the same as that in fed, wild-type mice. The mutant malic enzyme mRNA also were present in RNA from starved mice but at much lower concentrations. The mutation responsible for the Mod-1n phenotype is in the structural gene for malic enzyme. Undetectable synthesis of enzyme in the presence of a normal abundance of mRNA suggests either a protein product with an exceptionally short half-life or an altered mRNA that cannot be translated in liver.