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Journal of Bacteriology, January 2001, p. 500-511, Vol. 183, No. 2
National Institute of Animal Health, Tsukuba,
Ibaraki 305-0856,1 and Kenhoku
Livestock Hygiene Service Center, Mito, Ibaraki
310-0002,2 Japan
Received 17 July 2000/Accepted 24 October 2000
Different strains of Streptococcus suis serotypes 1 and
2 isolated from pigs either contained a restriction-modification (R-M) system or lacked it. The R-M system was an isoschizomer of
Streptococcus pneumoniae DpnII, which recognizes nucleotide
sequence 5'-GATC-3'. The nucleotide sequencing of the genes encoding
the R-M system in S. suis DAT1, designated
SsuDAT1I, showed that the SsuDAT1I gene region
contained two methyltransferase genes, designated ssuMA and
ssuMB, as does the DpnII system. The deduced
amino acid sequences of M.SsuMA and M.SsuMB
showed 70 and 90% identity to M.DpnII and
M.DpnA, respectively. However, the SsuDAT1I
system contained two isoschizomeric restriction endonuclease genes,
designated ssuRA and ssuRB. The deduced amino
acid sequence of R.SsuRA was 49% identical to that of
R.DpnII, and R.SsuRB was 72% identical to
R.LlaDCHI of Lactococcus lactis subsp.
cremoris DCH-4. The four SsuDAT1I genes
overlapped and were bounded by purine biosynthetic gene clusters in the
following gene order:
purF-purM-purN-purH-ssuMA-ssuMB-ssuRA-ssuRB-purD-purE. The
G+C content of the SsuDAT1I gene region (34.1%) was lower than that of the pur region (48.9%), suggesting horizontal
transfer of the SsuDAT1I system. No transposable element or
long-repeat sequence was found in the flanking regions. The
SsuDAT1I genes were functional by themselves, as they were
individually expressed in Escherichia coli. Comparison of
the sequences between strains with and without the R-M system showed
that only the region from 53 bp upstream of ssuMA to 5 bp
downstream of ssuRB was inserted in the intergenic sequence
between purH and purD and that the insertion
target site was not the recognition site of SsuDAT1I. No
notable substitutions or insertions could be found, and the structures
were conserved among all the strains. These results suggest that the
SsuDAT1I system could have been integrated into the
S. suis chromosome by an illegitimate recombination mechanism.
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.2.500-511.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Evidence for Horizontal Transfer of
SsuDAT1I Restriction-Modification Genes to the
Streptococcus suis Genome
*
Corresponding author. Mailing address: Laboratory of
Molecular Bacteriology, National Institute of Animal Health, 3-1-1 Kannondai, Tsukuba, Ibaraki 305-0856, Japan. Phone: 81 (298) 38-7743. Fax: 81 (298) 38-7907. E-mail:
sekizaki{at}niah.affrc.go.jp.
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