Investigation of a Novel DNase of Streptococcus suis Serotype 2

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Abstract
A secreted nuclease, SsnA, was identified in the virulent Streptococcus suis isolate SX332 and subsequently in each of the type strains of capsular serotypes 1 through 9. Screening of 258 porcine clinical isolates from surface (nasal mucosa or palatine tonsil) or internal (joint, brain or other internal organ) locations revealed a significant relationship ( P < 0.001) between expression of nuclease and isolation from an internal site. A 3,126-bp gene, ssnA , was identified from a phenotypically nuclease-negative pGh9:IS S1 insertion mutant, and analysis of the predicted SsnA sequence revealed a 35-amino-acid (aa) secretion signal sequence, a 22-aa DNA-binding domain, and a typical gram-positive cell wall sorting motif. A requirement of Ca 2+ and Mg 2+ for SsnA activity was determined, and the substrate specificity was found to be for single- and double-stranded linear DNA. Reverse transcription-PCR experiments revealed that ssnA is expressed throughout all stages of S. suis growth, and Western blots with porcine anti- S. suis immune sera against a recombinant, truncated SsnA derivative (rSsnA Δ ) confirmed that SsnA is expressed in vivo. Furthermore, anti-rSsnA Δ antibodies were sufficient to neutralize SsnA activity. Analyses of subcellular fractions of SX332 and derived mutants, on DNA-containing polyacrylamide gels and by Western blotting, suggest that SsnA is cell wall located.