A novel polysaccharide in the envelope of S. aureus influences the septal secretion of preproteins with a YSIRK/GXXS motif

Bacteria transport proteins across the plasma membrane to assemble their envelope, acquire nutrients, and establish appropriate interactions with their environment. The majority of these proteins are synthesized as precursors with a cleavable N-terminal signal sequence for recognition by the Sec machinery. In Staphylococcus aureus , a small subset of secreted precursors carries a YSIRK/GXXS motif. This motif provides a pre-translocation function by promoting the targeting of precursors to septal membranes, but the trans- acting factors that regulate such spatial distribution are not known. Here, we used immunofluorescence-microscopy to compare the spatial trafficking of Staphylococcal protein A (SpA), an abundant YSIRK/GXXS bearing precursor, between mutants of an arranged transposon library. This genetic search identified a cluster of five genes predicted to encode enzymes responsible for the synthesis of a novel surface polymer referred to as Staphylococcal surface carbohydrate, Ssc. Mutants in the ssc gene cluster no longer restrict the secretion of SpA into the cross-walls of S. aureus. ssc mutants replicate like wild-type bacteria unless grown in phosphate-limited conditions, and do not contribute to virulence when examined in a mouse model of bloodstream infection. Together, our observations suggest that S. aureus may encode a minor, phosphate-free carbohydrate, and propose a possible assembly pathway for this polymer. IMPORTANCE Gram-positive bacteria assemble peptidoglycan-linked polymers known as wall teichoic acids (WTA). Both Staphylococcus aureus and Bacillus subtilis elaborate WTAs made of poly-glycerol or poly-ribitol phosphates. WTAs contribute to cell shape maintenance, cation homeostasis, and resistance to antimicrobial compounds. Yet, B. subtilis replaces its phosphate-rich polymer with minor teichuronic acids whose functions remain elusive. S. aureus also encodes a minor wall polymer that may be required for growth under phosphate-limited condition. Here, we find that this polymer could help define the composition of the septal compartment, the site of cell division also used to recruit preproteins with a YSIRK/GXXS motif. Thus, the envelope of S. aureus may be more complex than previously thought with minor wall polymers contributing some discrete functions.