THE N-TERMINAL, DEHYDROGENASE CYCLOHYDROLASE DOMAIN OF YEAST CYTOPLASMIC TRIFUNCTIONAL C1-TETRAHYDROFOLATE SYNTHASE REQUIRES THE C-TERMINAL, SYNTHETASE DOMAIN FOR THE CATALYTIC ACTIVITY IN-VITRO

The yeast ADE3(1-333) gene which encodes a truncated protein containin g the N-terminal 5,10-methylene-tetrahydrofolate (THF) dehydrogenase ( D)/5,10-methenyl-THF cyclohydrolase (C) domain of cytoplasmic trifunct ional C1-THF synthase is able to complement all the phenotypes associa ted with ade3 mutations in vivo. However, expression of the ADE3(1-333 ) gene in an ade3 strain does not retain any D activity in vitro. Expr ession in a yeast ade3 strain of the ADE3(1-333) fused to the Escheric hia coli lacZ gene or to the yeast SER2 gene allows detection of D and C activities in vitro. These results indicate that the N-terminal DIC domain of C1-THF synthase requires the C-terminal 10-formyl-THF synth etase domain for stable catalytic activity in vitro.