Studies on identifying the allosteric binding sites of deoxycytidylate deaminase

Thymidine triphosphate, a negative regulator of deoxycytidylate deaminase, was found to bind covalently to this enzyme on exposure to UV light at 254 nM. The rate of half-maximal fixation was extremely rapid, occurring within 30 s and probably attaining a maximum of about 1 mol of dTTP fixed/mol of enzyme subunit. In contrast to the case of ribonucleotide reductase (Ericksson, S., Caras, I. W., and Martin, D. W., Jr. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 81-85) where the fixation of dTTP inactivated this enzyme, the activity of the deaminase was unaffected. The bound nucleotide could be released on exposure to UV 254 nm light in the presence of dCTP or dTTP but not dATP or dGTP. The enzyme-fixed nucleotide was found to remain with the larger of the two peptides released as a result of CNBr treatment of the labeled enzyme. Studies are in progress to define the location of this nucleotide, which will be aided greatly by our recent clarification of the complete amino acid sequence of T2-deoxycytidylate deaminase.