Modulation of specific binding of Lactobacillus casei dihydrofolate reductase to DNA by folinic acid.

Dihydrofolate reductase (DHFR) plays an essential role in intermediary metabolism, catalysing the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate, the latter serving as a carrier of onecarbon fragments in the biosynthesis of amino acids, thymidylate and purines [ 11. DHFR has been the subject of intense investigations as it is the target of important anti-bacterial and anti-neoplastic drugs such as trimethoprim and methotrexate. It has been demonstrated that DHFR from Lactobacillus casei has affinity for double-stranded DNA [2]. A small difference was observed in the nitrocellulose filter binding curves obtained for the binding of DHFR to pBR 322 and pWDLcB1 DNA, the latter differing from the former only in the presence of a 2.9 kb insert containing the DHFR structural gene from a methotrexate-resistant strain of L. casei, suggesting that DHFR binds specifically to a site at or near its own structural gene. Here, we demonstrate that specific binding of DHFR to DNA is modulated by the tetrahydrofolate analogue folinic acid but not by the coenzyme NADPH, in contrast to non-specific binding which is unaffected by either folinic acid or NADPH.