Purification of the MutX protein of Streptococcus pneumoniae, a homologue of Escherichia coli MutT. Identification of a novel catalytic domain for nucleoside triphosphate pyrophosphohydrolase activity.

The mutX gene of Streptococcus pneumoniae, a homologue of the Escherichia coli mutT mutator gene (Méjean, V., Salles, C., Bullions, L. C., Bessman, M. J., and Claverys, J.-P. (1993) Mol. Microbiol. 11, 323-330) has been cloned into an expression vector, and its gene product, the MutX protein, has been purified to apparent homogeneity. Like MutT, the pure MutX protein hydrolyzes all of the canonical nucleoside triphosphates at different rates with a preference for dGTP, yielding nucleoside monophosphates and inorganic pyrophosphate. Despite this similarity in enzymatic activity, the two proteins have notably dissimilar primary and quaternary structures. They share only a small region of amino acid homology, and under the same conditions in which MutT exists as a monomer in solution, MutX behaves as a trimer. The small region of conserved amino acid sequence most likely identifies a protein domain responsible for the novel nucleoside triphosphate pyrophosphohydrolase activity shared by the two enzymes, and by another protein of unknown function, the product of the E. coli orf17 gene (Takahagi, M., Iwasaki, H., Nakata, A., and Shinegawa, H. (1991) J. Bacteriol. 173, 5747-5753).