Affiliation:
1. National Institute of Animal Health, Tsukuba, Ibaraki, Japan
Abstract
ABSTRACT
Many surface proteins which are covalently linked to the cell wall of gram-positive bacteria have a consensus C-terminal motif, Leu-Pro-X-Thr-Gly (LPXTG). This sequence is cleaved, and the processed protein is attached to an amino group of a cross-bridge in the peptideglycan by a specific enzyme called sortase. Using the type strain of
Streptococcus suis
, NCTC 10234, we found five genes encoding proteins that were homologous to sortases of other bacteria and determined the nucleotide sequences of the genetic regions. One gene, designated
srtA
, was linked to
gyrA
, as were the sortase and sortase-like genes of other streptococci. Three genes, designated
srtB
,
srtC
, and
srtD
, were tandemly clustered in a different location, where there were three segments of directly repeated sequences of approximately 110 bp in close vicinity. The remaining gene, designated
srtE
, was located separately on the chromosome with a pseudogene which may encode a transposase. The deduced amino acid sequences of the five Srt proteins showed 18 to 31% identity with the sortases of
Streptococcus gordonii
and
Staphylococcus aureus
, except that SrtA of
S. suis
had 65% identity with that of
S. gordonii
. Isogenic mutants deficient for
srtA
,
srtBCD
, or
srtE
were generated by allelic exchanges. The protein fraction which was released from partially purified cell walls by digestion with
N
-acetylmuramidase was profiled by two-dimensional gel electrophoresis. More than 15 of the protein spots were missing in the profile of the
srtA
mutant compared with that of the parent strain, and this phenotype was completely complemented by
srtA
cloned from
S. suis
. Four genes encoding proteins corresponding to such spots were identified and sequenced. The deduced translational products of the four genes possessed the LPXTG motif in their C-terminal regions. On the other hand, the protein spots that were missing in the
srtA
mutant appeared in the profiles of the
srtBCD
and
srtE
mutants. These results provide evidence that the cell wall sorting system involving
srtA
is also present in
S. suis
.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
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