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

## TL;DR

This study investigates how two substrate-binding proteins in Gardnerella swidsinskii interact with maltose and related sugars, revealing their binding affinities and expression patterns.

## Contribution

The paper identifies and compares two substrate-binding proteins in G. swidsinskii, showing their similar ligand affinities and expression dynamics.

## Key findings

- Both SBP genes are expressed as a polycistronic transcript, with musE1346 transcripts being more abundant.
- Both SBPs show high affinity for maltose, maltotriose, and maltotetraose but lower affinity for longer malto-oligosaccharides.
- Expression levels of the SBP genes do not change significantly in media with glycogen or maltotriose.

## Abstract

Glycogen and its breakdown products, maltose and malto-oligosaccharides, are important carbon sources for vaginal bacteria including Gardnerella species. MusEFGKI transport systems for maltose and malto-oligosaccharides have been identified in all Gardnerella species; however, unlike in other species, the Gardnerella swidsinskii 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 G. swidsinskii relative to other species with only one SBP. Our objectives were to determine if both genes are expressed in G. swidsinskii and compare the specificity and affinity of G. swidsinskii MusE SBPs for glycogen breakdown products. Gene expression analysis showed the presence of a polycistronic transcript spanning both SBP encoding genes; however, musE1346 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 (Kd 10−6 to 10−7 M) and much lower affinities to maltopentaose and maltohexaose (Kd 10−3 to 10−4 M). Our results demonstrate that the affinities of G. swidsinskii MusE SBPs for maltose and malto-oligosaccharides are similar under the same experimental conditions.