RAT GENOMIC STRUCTURE OF AMIDOPHOSPHORIBOSYLTRANSFERASE, CDNA SEQUENCE OF AMINOIMIDAZOLE RIBONUCLEOTIDE CARBOXYLASE, AND CELL CYCLE-DEPENDENT EXPRESSION OF THESE 2 PHYSICALLY LINKED GENES

Genomic structure of rat amidophosphoribosyltransferase (ATase; EC 2.4 .2.14), which catalyzes the first committed step in de novo purine nuc leotide synthesis, was determined by polymerase chain reaction (PCR)-b ased methods. There are 11 exons and all exon-intron boundaries were c onserved among rat, human, and chicken ATase genes. A rat aminoimidazo le ribonucleotide carboxylase (AIRC) cDNA encoding a bifunctional enzy me of AIRC (EC 4.1.1.21) at step 6 and SAICAR synthetase (EC 6.3.2.6) at step 7 in de novo purine nucleotide synthesis was cloned and sequen ced. The size of the cloned rat AIRC cDNA was 1329 bp, and amino acid identity with human and chicken AIRC was 96 and 85%, respectively. The intergenic sequence using a phage clone and the PCR product disclosed that ATase and AIRC genes are physically linked with the 736 bp seque nce between the translation start sites, and the determination of the transcriptional start sites by the primer extension assay for these ge nes disclosed that distance between the two major transcriptional star t sites is 585 bp. The amount of mRNAs of both genes showed approx. 5- 6-fold increase in G(1)/S phase of the cell cycle over those in G(0) p hase in synchronized rat 3Y1 fibroblasts.