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1 Department of Diagnostic and Biological Sciences, School of Dentistry, Medical School, University of Minnesota, Minneapolis, MN 55455, USA
2 Department of Genetics, Cell Biology and Development, Medical School, University of Minnesota, Minneapolis, MN 55455, USA
3 Department of Oral Biology, Joint Health Science Center, University of Florida, Gainesville, FL 32611, USA
4 Mucosal and Vaccine Research Center, Minneapolis VA Medical Center, Minneapolis, MN 55417, USA
Correspondence
Mark C. Herzberg
mcherzb{at}umn.edu
Sortase A (SrtA) is required for cell-wall anchoring of LPXTG-containing Gram-positive surface proteins. It was hypothesized, therefore, that disruption of the srtA gene would alter surface anchoring and functions of target LPXTG motif-bearing SspA and SspB proteins of Streptococcus gordonii. Mutant strains in srtA (V288srtA–, DL1srtA–) were constructed in S. gordonii V288 (wtV288) and DL1 (wtDL1). When compared to wtV288, the V288srtA– mutant showed decreased biofilm formation on polystyrene, and reduced binding to immobilized purified salivary agglutinin (BIAcore analysis). The wtV288 and V288srtA– strains were similar in ultrastructure, but immunogold-labelled SspA/SspB surface expression was reduced on the V288srtA– mutant. DL1srtA– was also complemented to obtain DL1srtA+. From the wild-type strains (wtV288, wtDL1), srtA– mutants (V288srtA–, DL1srtA–), and the complemented mutant (DL1srtA+), cytoplasmic, cell-wall and released extracellular protein fractions were isolated. Each fraction was analysed by SDS-PAGE and immunoblotting with anti-P1. Spent medium from srtA– mutant cells contained over-represented proteins, including SspA/SspB (P1 antigen). Mutants showed less P1 on the cell surface than wild-types, as estimated using whole-cell ELISA, and no P1 appeared in the cytoplasmic fractions. Expression of several adhesin genes (sspA/B, cshA/B, fbpA) was generally upregulated in the mutants (V288srtA–, DL1srtA–), but restored to wild-type levels in DL1srtA+. These data therefore imply that in addition to its role in processing LPXTG-containing adhesins, sortase A has the novel function of contributing to transcriptional regulation of adhesin gene expression.
These authors contributed equally to this work.
Present address: Novartis Vaccines, Centro Ricerche, Via Fiorentina 1, 53100 Siena, Italy.
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