A 3RD FAMILY OF ALLELIC HSD GENES IN SALMONELLA-ENTERICA - SEQUENCE COMPARISONS WITH RELATED PROTEINS IDENTIFY CONSERVED REGIONS IMPLICATEDIN RESTRICTION OF DNA

Salmonella enterica serovar blegdam has a restriction and modification system encoded by genes linked to serB. We have cloned these genes, p utative alleles of the hsd locus of Escherichia coli K-12, and confirm ed by the sequence similarities of flanking DNA that the hsd genes of S. enterica serovar blegdam have the same chromosomal location as thos e of E. coli K-12 and Salmonella enterica serovar typhimurium LT2. The re is, however, no obvious similarity in their nucleotide sequences, a nd while the gene order in S. enterica serovar blegdam is serB hsdM, S and R, that in E. coli K-12 and S. enterica serovar typhimurium LT2 i s serB hsdR, M and S. The hsd genes of S. enterica serovar blegdam ide ntify a third family of serB-linked hsd genes (type ID). The polypepti de sequence predicted from the three hsd genes show some similarities (18-50% identity) with the polypeptides of known and putative type I r estriction and modification systems; the highest levels of identity ar e with sequences of Haemophilus influenzae Rd. The HsdM polypeptide ha s the motifs characteristic of adenine methyltransferases. Comparisons of the HsdR sequence with those for three other families of type I sy stems and three putative HsdR polypeptides identify two highly conserv ed regions in addition to the seven proposed DEAD-box motifs.