Song J M, Rabinowitz J C
Department of Molecular and Cell Biology, University of California, Berkeley 94720.
Proc Natl Acad Sci U S A. 1993 Apr 1;90(7):2636-40. doi: 10.1073/pnas.90.7.2636.
The protein product of the ADE3 gene of the yeast Saccharomyces cerevisiae has been identified as the cytoplasmic trifunctional C1-tetrahydrofolate (THF) synthase, which possesses 10-formyl-THF synthetase (EC 6.3.4.3), 5,10-methenyl-THF cyclohydrolase (EC 3.5.4.9), and 5,10-methylene-THF dehydrogenase (EC 1.5.1.5) activities. However, it has been suggested that the ADE3-encoded C1-THF synthase does not play a role in providing the enzymes involved in the generation of one-carbon intermediates in the biosynthesis of the purine bases but functions in maintaining the structural integrity of the enzyme complex involved in purine biosynthesis [Barlowe, C. K. & Appling, D. A. (1990) Mol. Cell. Biol. 10, 5679-5687]. This hypothesis is based on their finding that the presence of the full-length ADE3 C1-THF synthase, whether catalytically active or not, is correlated with the Ade+ phenotype. In contrast to their results, our deletion analysis of the ADE3 gene indicates that the presence of either the synthetase or dehydrogenase/cyclohydrolase domains of C1-THF synthase is enough to complement the adenine requirement in ade3 strains. These results are also consistent with those obtained in heterologous expression of spinach and Clostridium acidiurici monofunctional synthetases in ade3 strains. Heterologous expression studies show that the high synthetase activity may be correlated with the increased growth in medium lacking adenine. These results suggest that the catalytic activity of the C1-THF synthase is involved in purine biosynthesis.
酿酒酵母ADE3基因的蛋白质产物已被鉴定为细胞质三功能C1-四氢叶酸（THF）合酶，它具有10-甲酰基-THF合成酶（EC 6.3.4.3）、5,10-亚甲基-THF环水解酶（EC 3.5.4.9）和5,10-亚甲基-THF脱氢酶（EC 1.5.1.5）活性。然而，有人提出，ADE3编码的C1-THF合酶在嘌呤碱基生物合成中提供参与一碳中间体生成的酶方面不起作用，而是在维持嘌呤生物合成中涉及的酶复合物的结构完整性方面发挥作用[巴洛，C.K. & 阿普林，D.A.（1990年）《分子与细胞生物学》10，5679 - 5687]。这一假设基于他们的发现，即全长ADE3 C1-THF合酶的存在，无论其是否具有催化活性，都与Ade+表型相关。与他们的结果相反，我们对ADE3基因的缺失分析表明，C1-THF合酶的合成酶或脱氢酶/环水解酶结构域的存在足以补充ade3菌株对腺嘌呤的需求。这些结果也与在ade3菌株中菠菜和尿酸梭菌单功能合成酶的异源表达所获得的结果一致。异源表达研究表明，高合成酶活性可能与在缺乏腺嘌呤的培养基中生长增加相关。这些结果表明，C1-THF合酶的催化活性参与嘌呤生物合成。
