Distribution of the SsuDAT1I restriction-modification system among different serotypes of Streptococcus suis

Abstract

The SsuDAT1I restriction-modification (R-M) system, which contains two methyltransferases and two restriction endonucleases with recognition sequence 5'-GATC-3', was first found in a field isolate of Streptococcus suis serotype 2. Isoschizomers of the R-M system were found in the same locus between purH and purD in a field isolate of serotype 1/2 and the reference strains of serotypes 3, 7, 23, and 26 among 29 strains of different serotypes examined in this study. The R-M gene sequences in serotypes 1/2, 3, 7, and 23 were very similar to those of SsuDAT1I, whereas those in serotype 26 were less similar. These results indicate intraspecies recombination among them and genetic divergence through their evolution.

FIG. 1

Restriction cleavage activities of the crude extracts using unmethylated pUC19 as the substrate. Lane numbers represent the serotypes of the strains from which the crude extracts were prepared. M, pUC19 digested with MboI; S, 100-bp ladder size standards (GIBCO/BRL).

FIG. 2

Genomic Southern hybridization of S. suis strains probed with 3,503-bp fragment of the SsuDAT1I gene region. DNA was digested with PvuII and separated by agarose gel electrophoresis. Lane numbers represent the serotypes of the strains used. N, NCTC10234; D, DAT1. Molecular sizes of the signals that appeared are indicated on the left in kilobases.

FIG. 3

Sequence alignment of M.SsuDAT1IA (MA.DAT1), M.Ssu4109IA (MA.4109), M.DpnII, and M.LlaA (A); M.SsuDAT1IB (MB.DAT1), M.Ssu4109IB (MB.4109), M.DpnA, and M.LlaB (B); and R.SsuDAT1IA (RA.DAT1), R.Ssu4109IA (RA.4109), R.DpnII, R.SsuDAT1IB (RB.DAT1), R.Ssu4109IB (RB.4109), and R.LlaDCHI (R.LlaC) (C). Black background, amino acids identical among all the sequences aligned; dashes, gaps in the aligned sequences; dots, positions where the amino acids of SsuDAT1I are different from those of Ssu4109I. DDBJ/EMBL/GenBank accession numbers of the sequences of the SsuDAT1I, DpnII, and LlaDCHI systems are AB045609 (32), M14339 (20), and U16027 (23), respectively.