Ligand-binding properties of substrate binding proteins of a maltose uptake system in Gardnerella swidsinskii

Glycogen and its breakdown products, maltose and malto-oligosaccharides, are important carbon sources for vaginal bacteria including <i>Gardnerella</i> species. MusEFGKI transport systems for maltose and malto-oligosaccharides have been identified in all <i>Gardnerella</i> species; however, unlike in other species, the <i>Gardnerella swidsinskii</i> operon encodes two substrate-binding proteins (SBPs) (MusE1345, MusE1346, ~60% amino acid identity). Two SBPs could allow binding of additional ligands, providing a competitive advantage to <i>G. swidsinskii</i> relative to other species with only one SBP. Our objectives were to determine if both genes are expressed in <i>G. swidsinskii</i> and compare the specificity and affinity of <i>G. swidsinskii</i> MusE SBPs for glycogen breakdown products. Gene expression analysis showed the presence of a polycistronic transcript spanning both SBP encoding genes; however, <i>musE1346</i> transcripts were more abundant, likely due to the presence of an additional promoter identified in the intergenic region. No difference in the relative expression of either gene was observed in isolates grown in media supplemented with glycogen or maltotriose. Predicted structures of both SBPs were highly similar and characteristic of previously characterized maltose-binding proteins. Both proteins had a high affinity for maltose, maltotriose and maltotetraose (<i>K</i> _d 10^-6 to 10^-7 M) and much lower affinities to maltopentaose and maltohexaose (<i>K</i> _d 10^-3 to 10^-4 M). Our results demonstrate that the affinities of <i>G. swidsinskii</i> MusE SBPs for maltose and malto-oligosaccharides are similar under the same experimental conditions.