| Pyrimidine nucleotides are abundant molecules with essential functions in a multitude of biochemical processes. They are the building blocks for the direct synthesis of DNA and RNA that function in information storage and retrieval within the cell, but they were also directly involved in carbon metabolism providing energy-rich precursors for a lot of synthetic reactions such as UDP-glucose as a precursor in sucrose or cell wall biosynthesis.De novo pyrimidine biosynthesis, which is also referred to as the "orotate pathway", is one of the most important methoblism pathways in plants. In contrast to the wealth of information about de novo pyrimidine pathway in animals and microorganisms, little is known about de novo pyrimidine pathway in plants.Dihydroorotate dehydrogenase (DHODH) is the fourth step catalyzing the oxidation of dehydroorotate to orotate in the biosynthesis of UMP, the precursor of all other pyrimidine nucleotides. In order to isolate DHODH cDNAs from rice (Oryza sativa L.), Arabidopsis thaliana DHODH gene (GenBank accession number: AF454729) and rice DHODH partial cDNA sequence (GenBank accession number: AF486618) were used as query probes to search rice genome database in GenBank through BLAST algorithm program. Two genomic sequences were found to be homologous to probes, respectively. Two full-length cDNAs were assembled using FGENESH program. By RT-PCR, two cDNAs were isolated from rice seedlings, and designated as OsDHODH1 and OsDHODH2 with accession number DQ102485 and DQ269457 in GenBank, respectively. It should be the first report that both DHODH genes were cloned from one plant organism.Regarding genomic structure and phylogenetic analysis here, we found the relationships among these sequences are not completely discordant with the accepted organismal phylogeny. Althougth this alignment supports the classification of the DHODHs into families 1 and 2, there are two DHODHs in the same organism and are classified into families 1 and 2, respectively, which is the first reported in the higher plants. For the possible origin of the membrane-bound and cytosolic enzymes, the hypothetical ancestor of... |
