CHARACTERIZATION OF THE MUTX GENE OF STREPTOCOCCUS-PNEUMONIAE AS A HOMOLOG OF ESCHERICHIA-COLI MUTT, AND TENTATIVE DEFINITION OF A CATALYTIC DOMAIN OF THE DGTP PYROPHOSPHOHYDROLASES

We show that deletion of a gene of Streptococcus pneumoniae, which we call mutX, confers a mutator phenotype to resistance to streptomycin. Analysis of the DNA sequence changes that occurred in several streptom ycin-resistant mutants showed that mutations are unidirectional AT to CG transversions. The mutX gene is located immediately downstream of t he previously identified ung gene and genetic evidence suggests that t he two genes are co-ordinately regulated. Nucleotide sequence determin ation reveals that the mutX gene encodes a 17870 Da protein (154 resid ues) which exhibits significant homology with the MutT protein of Esch erichia coli, a nucleoside triphosphatase (dGTP pyrophosphohydrolase). The mutX gene complements the E. coli mutT mutator phenotype when int roduced on a plasmid. Site-directed mutagenesis and analysis of nitros oguanidine-induced mutT mutants suggest that a small region of high ho mology between the two proteins (61% identity over 23 residues) is par t of the catalytic site of the nucleoside triphosphatase. Computer sea rching for sequence homology to MutX uncovered a second E. coli protei n, the product of orf17, a gene of unknown function located near the r uvC gene. The region of high homology between MutX and MutT is also co nserved in this protein, which raises the interesting possibility that the orf17 gene plays some role in determining mutation rates in E. co li. Finally, a small set of proteins, including a family of virus-enco ded proteins and two evolutionarily conserved proteins encoded by an a ntisense transcript from the Xenopus laevis and human bFGF genes, were also found to harbour significant homology to this highly conserved r egion.