Genetic and biochemical properties of streptococcal NAD-glycohydrolase inhibitor

The gene encoding streptolysin O (slo), a cytolysin of hemolytic streptococci, is transcribed polycistronically from the promoter of the preceding NAD-glycohydrolase (NADase) gene (nga). Between nga and slo, a putative open reading frame (orf1) is located whose function has been totally unknown. Present investigation demonstrated that the orf1 encodes a protein designated as streptococcal NADase inhibitor (SNI). From its nucleotide sequence, SNI was inferred to comprise 161 amino acid residues and the deduced molecular weight was 18,800. This protein was detectable only within cells. Coexpression of SNI was essential for production of streptococcal NADase, and NADase precursor existed as an inactive complex with SNI, in recombinant Escherichia coli. Monomeric NADase and SNI rapidly formed in vitro a stable heterodimer complex in the ratio 1:1, resulting in complete suppression of the hydrolase activity. Unlike other bacterial NADase inhibitors, SNI was thermostable. This protein, coexpressed and complexed with NADase, may protect the producer cocci from exhaustion of NAD.     

The NAD-glycohydrolase (nga) gene of Streptococcus pyogenes

The gene for NAD-glycohydrolase (nga) of group A streptococci (Streptococcus pyogenes) was identified and shown to be located immediately adjacent to the gene for streptolysin O (slo). The nga gene contains 1341 base pairs and encodes a protein of 447 amino acids, including an N-terminal signal peptide. Results from analysis with the polymerase chain reaction indicated that the nga gene is present in all of the strains tested. Functional extracellular NAD-glycohydrolase, also known as NADase, was detected among a wide variety of clinical isolates and known laboratory strains and shown to be present in 72% of 100 strains examined. In contrast, 92% of strains isolated from patients with invasive streptococcal infections were positive for NADase production.