Streptococcus pyogenes nuclease A (SpnA) mediated virulence does not exclusively depend on nuclease activity

Streptococcus pyogenes, or Group A Streptococcus (GAS), is a human pathogen that causes a wide range of diseases, including pharyngitis, necrotizing fasciitis and toxic shock syndrome. The bacterium produces a large arsenal of virulence factors, including the cell wall-anchored Streptococcus pyogenes nuclease A (SpnA), which facilitates immune evasion by degrading the DNA backbone of neutrophil extracellular traps. SpnA consists of a C-terminal endo/exonuclease domain and a N-terminal domain of unknown function. Recombinant SpnA mutants were generated by alanine conversion of selected residues that were predicted to play a role in the enzymatic activity and tested for their ability to degrade DNA. A GAS spnA deletion mutant was complemented with a plasmid-borne catalytic site mutant and analyzed for virulence in a Galleria mellonella (wax moth) infection model. Several predicted residues were experimentally confirmed to play a role in SpnA enzymatic activity. These include Glu592, Arg696, His716, Asp767, Asn769, Asp810 and Asp842. Complementation of a GAS spnA deletion mutant with a spnA H716A mutant gene partially restored virulence in wax moth larvae, whereas complementation with the spnA wt gene completely restored activity. Furthermore, complementation with a secreted form of SpnA showed reduced virulence. Our results show that abolishing the enzymatic activity of SpnA only partially reduces virulence suggesting that SpnA has an additional virulence function, which might be located on the N-terminal domain. Furthermore, cell wall-anchoring of SpnA results in higher virulence compared to secreted SpnA, probably due to a higher local density of the enzyme.