The molecular basis for a cytosolic malic enzyme null mutation. Malic enzyme mRNA from MOD-1 null mice contains an internal in-frame duplication that extends the coding sequence by 522 nucleotides.

Many tissues from wild type mice express cytosolic malic enzyme activity and contain two mRNAs (2.0 and 3.1 kilobases (kb)) that encode a single 64-kDa malic enzyme subunit polypeptide. MOD-1 null mutant mice lack cytosolic malic enzyme activity but express 2.5- and 3.6-kb mRNAs that hybridize with wild type malic enzyme cDNAs and are induced in liver by a starvation/carbohydrate refeeding regimen. To investigate the basis of the MOD-1 null mutation, a lambda gt11 cDNA library was constructed using mRNA from the livers of induced MOD-1 null mice as a template. A recombinant phage with a 2-kb insert was isolated by screening with wild type malic enzyme cDNA probes. The subcloned insert exhibited an atypical (non-wild type) restriction pattern and was subjected to sequence analysis. MOD-1 null malic enzyme cDNA contains an internal tandemly duplicated sequence that corresponds to nucleotides 1027-1548 in the coding region of wild type murine malic enzyme cDNA (Bagchi, S., Wise, L. S., Brown, M. L., Bregman, D., Sul, H. S., and Rubin, C. S. (1987) J. Biol. Chem. 262, 1558-1565). An open reading frame is retained throughout the duplicated sequence. The discovery of a 522-nucleotide in-frame duplication accounts for the increased size of MOD-1 null malic enzyme mRNAs and suggests that a variant malic enzyme polypeptide that is 19 kDa larger than the wild type subunit might be found in mutant mice. Western immunoblot analysis disclosed that MOD-1 null liver cytosol contains an 82-kDa protein that is recognized by anti-malic enzyme antibodies. Under stringent conditions, an anti-sense 32P-oligonucleotide that spans the abnormal junction between the reiterated sequences hybridized with the 2.5 and 3.6-kb MOD-1 null malic enzyme mRNAs but failed to form stable complexes with wild type malic enzyme mRNAs. Thus, both MOD-1 null malic enzyme mRNAs contain the duplication deduced from cDNA sequence analyses. The MOD-1 null mutation might originate from an unequal crossover between homologous regions of two different introns in the malic enzyme gene, thereby causing the duplication of one or more exons.