Cloning and sequence of the human type II IMP dehydrogenase gene.

Human type II inosine 5'-monophosphate dehydrogenase (EC 1.1.1.205) is the rate-limiting enzyme in de novo guanine nucleotide biosynthesis. Regulated inosine 5'-monophosphate dehydrogenase activity is associated with cellular proliferation, transformation, and differentiation. We cloned and sequenced the entire gene for type II inosine 5'-monophosphate dehydrogenase and here provide details regarding the organization of the gene and its transcriptional start sites. The gene spans approximately 5 kb and is disrupted by 12 introns. The transcriptional start sites were determined by S1 nuclease mapping to be somewhat heterogeneous but predominated at 102 and 85 nucleotides from the translational initiation codon.

Regulation of IMP dehydrogenase gene expression by its end products, guanine nucleotides.

To study the regulation of IMP dehydrogenase (IMPDH), the rate-limiting enzyme of guanine nucleotide biosynthesis, we examined the effects of nucleosides, nucleotides, nucleotide analogs, or the IMPDH inhibitor mycophenolic acid (MPA) on the steady-state levels of IMPDH mRNA. The results indicated that IMPDH gene expression is regulated inversely by the intracellular level of guanine ribonucleotides. We have shown that treatment with guanosine increased the level of cellular guanine ribonucleotides and subsequently reduced IMPDH steady-state mRNA levels in a time- and dose-dependent manner. Conversely, MPA treatment diminished the level of guanine ribonucleotides and increased IMPDH mRNA levels. Both of these effects on the steady-state level of IMPDH mRNA could be negated by cotreatment with guanosine and MPA. The down regulation of IMPDH gene expression by guanosine or its up regulation by MPA was not due to major changes in transcriptional initiation and elongation or mRNA stability in the cytoplasm but rather was due to alterations in the levels of the IMPDH mRNA in the nucleus. These results suggest that IMPDH gene expression is regulated by a posttranscriptional, nuclear event in response to fluctuations in the intracellular level of guanine ribonucleotides.