Nucleotide sequence of the hexA gene for DNA mismatch repair in Streptococcus pneumoniae and homology of hexA to mutS of Escherichia coli and Salmonella typhimurium

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Abstract
The Hex system of heteroduplex DNA base mismatch repair operates in Streptococcus pneumoniae after transformation and replication to correct donor and nascent DNA strands, respectively. A functionally similar system, called Mut, operates in Escherichia coli and Salmonella typhimurium. The nucleotide sequence of a 3.8-kilobase segment from the S. pneumoniae chromosome that includes the 2.7-kilobase hexA gene was determined. An open reading frame that could encode a 17-kilodalton polypeptide (OrfC) was located just upstream of the gene encoding a polypeptide of 95 kilodaltons corresponding to HexA. Shine-Dalgarno sequences and putative promoters were identified upstream of each protein start site. Insertion mutations showed that only HexA functioned in mismatch repair and that the promoter for hexA transcription was located within the OrfC-coding region. The HexA polypeptide contains a consensus sequence for ATP- or GTP-binding sites in proteins. Comparison of the entire HexA protein sequence to that of MutS of S. typhimurium, which was determined by Haber et al. in the accompanying paper (L. T. Haber, P. P. Pang, D. I. Sobell, J. A. Mankovitch, and G. C. Walker, J. Bacteriol. 170:197-202, 1988), showed the proteins to be homologous, inasmuch as 36% of their amino acid residues were identical. This homology indicates that the Hex and Mut systems of mismatch repair evolved from an ancestor common to the gram-positive streptococci and the gram-negative enterobacteria. It is the first direct evidence linking the two systems.