A Conserved Streptococcal Virulence Regulator Controls the Expression of a Distinct Class of M-Like Proteins.

subspecies (SEZ) are group C streptococci that are important pathogens of economically valuable animals such as horses and pigs. Here, we found that many SEZ isolates bind to a monoclonal antibody that recognizes poly-acetylglucosamine (PNAG), a polymer that is found as a surface capsule-like structure on diverse microbes. A fluorescence-activated cell sorting-based transposon insertion sequencing (Tn-seq) screen, coupled with whole-genome sequencing, was used to search for genes for PNAG biosynthesis. Surprisingly, mutations in a gene encoding an M-like protein, , and the adjacent transcription factor, designated , rendered strains PNAG negative. SezV was required for expression and transcriptome analysis showed that SezV has a small regulon. SEZ strains with inactivating mutations in either or were highly attenuated in a mouse model of infection. Comparative genomic analyses revealed that linked and homologues are present in all SEZ, subspecies (SEE), and M18 group A streptococcal (GAS) genomes in the database, but not in other streptococci. The antibody to PNAG bound to a wide range of SEZ, SEE, and M18 GAS strains. Immunochemical studies suggest that the SzM protein may be decorated with a PNAG-like oligosaccharide although an intact oligosaccharide substituent could not be isolated. Collectively, our findings suggest that the and loci define a subtype of virulent streptococci and that an antibody to PNAG may have therapeutic applications in animal and human diseases caused by streptococci bearing SzM-like proteins. M proteins are surface-anchored virulence factors in group A streptococci, human pathogens. Here, we identified an M-like protein, SzM, and its positive regulator, SezV, in subspecies (SEZ), an important group of pathogens for domesticated animals, including horses and pigs. SzM and SezV homologues were found in the genomes of all SEZ and subspecies and M18 group A streptococcal strains analyzed but not in other streptococci. Mutant SEZ strains lacking either or were highly attenuated in a mouse model of infection. Collectively, our findings suggest that SezV-related regulators and the linked SzM family of M-like proteins define a new subset of virulent streptococci.