Uridine monophosphate (UMP) synthase is a bifunctional enzyme catalyzing the last two steps of de novo pyrimidine biosynthesis, orotate phosphoribosyltransferase (OPRT) and orotidine-5'-monophosphate decarboxylase (ODC). Loss of either enzymatic activity results in hereditary orotic aciduria, a rare autosomal recessive disorder characterized by retarded growth, anemia, and excessive urinary excretion of orotic add. We have isolated the UMP synthase chromosomal gene from a λEMBL-3 human genomic library and report a single-copy gene spanning ~15 kb. The UMP synthase genomic structure encodes six exons ranging in size from 115 bp to 672 bp, and all splicing junctions adhere to the canonical GT/AG rule. Cognate promoter elements implicated in glucocorticoid- and cAMP-mediated regulation as well as in liver-, myeloid-, and lymphocyte-specific expression are located within the 5' flanking sequence. Molecular investigation of UMP synthase deficiency in a Japanese orotic aciduria patient revealed mutations R96G (A-to-G transition; nt 286) and G429R (G-to-C transversion; nt 1285) in one allele and V109G (T-to-G transversion; nt 326) in the other allele. Expression of human UMP synthase cDNAs containing these mutations in pyrimidine auxotrophic Escherichia coli and in recombinant baculovirus- infected $f21 cells demonstrates impaired activity presumably associated with the urinary orotic acid substrate accumulations observed in vivo. We further establish the identity of two polymorphisms, G213A (v = .26) and 440Gpoly (v = .27) located in exons 3 and 6, respectively, which did not significantly compromise either OPRT or ODC function.
|Number of pages||15|
|Journal||American Journal of Human Genetics|
|Publication status||Published - 03-1997|
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