Characterization of the rat cytoplasmic C1-tetrahydrofolate synthase gene and analysis of its expression in liver regeneration and fetal development.

The eukaryotic trifunctional enzyme, C(1)-tetrahydrofolate (THF) synthase, interconverts folic acid derivatives between various oxidation states and is critical for normal cellular function, growth, and differentiation. Using a rat C(1)-THF synthase cDNA and synthetic oligonucleotides, the rat C(1)-THF synthase gene was isolated and characterized. The gene consists of 28 exons and spans 67.5 kbp. Primer extension, RNase protection, and rapid amplification of cDNA ends (RACE) experiments indicate the presence of multiple transcription start points (tsp) within a 250-bp window located between 50 and 300 bp upstream from the start codon. The 5' flanking region is devoid of a TATA consensus sequence motif, but putative regulatory elements, including NF-kappabeta, HNF-4alpha1, RARalpha1, C/EBP, and PPAR are present in the promoter region. The 5' flanking region also contains two sets of tetranucleotide repeats and two short interspersed nuclear elements (SINES). The initial 2500 bp of 5' flanking sequences of the rat and mouse cytoplasmic C(1)-THF synthase genes share 70% identity. However, comparison with the human gene from the Human Genome Data Bank revealed no significant homology in the 5' flanking region. The gene structure characterization led to the identification of a pseudogene that is 94% identical to the C(1)-THF synthase gene and probably diverged 10-12 million years ago. In addition, the gene expression patterns of C(1)-THF synthase were investigated during liver regeneration and liver and kidney organogenesis, two highly regulated events. In both processes, C(1)-THF synthase expression correlated with increased nucleotide metabolism. This pattern suggests that the gene is regulated in response to changes in the demand for folate-dependent one-carbon units.