Chitin — a polymer of N-acetyl glucosamine (GlcNAc) — is a highly abundant biological molecule and is present in huge amounts in marine ecosystems in the cuticles of zooplankton. Its degradation by ocean-dwelling bacteria is essential for the cycling of nutrients, and how these microorganisms sense and respond to chitin has long been a matter of interest. Work from Saul Roseman's laboratory now reveals that two vibrio species use a two-component regulatory system to detect chitin and activate the genes that break it down.

Chitin degradation by Vibrio furnissii and Vibrio cholerae involves dozens of genes, the expression of which Roseman and his colleague, Xibing Li, now show to be controlled by a two-component regulatory system. They used transposon mutagenesis to identify the sensor component of this system by selecting for V. furnissii mutants that were able to grow on GlcNAc but were unable to degrade chitin or produce any of the proteins required for its degradation. The chiS gene was found to encode the V. furnissii sensor component and the homologous gene from V. cholerae was identified. The ChiS protein comprises a periplasmic domain that senses the environmental signal and a cytoplasmic domain that transmits this information to activate gene expression.

These studies suggest a model in which ChiS is bound and locked into an inactive state by a periplasmic chitin-oligosaccharide-binding protein (CBP) in the absence of chitin oligosaccharides. According to this model, in the presence of (GlcNAc)n (which is produced by a chitinase secreted by the bacteria), ChiS is released by CBP and activates gene expression.

To test this, the authors used an assay that measures the activity of β-N-acetylglucosaminidases, enzymes in the chitin degradation pathway. In wild-type V. cholerae, these enzymes are only induced in the presence of (GlcNAc)2, and this activity is abolished if chiS is deleted, consistent with a role of chiS in activating genes involved in chitin catabolism. However, when the gene encoding CBP was deleted, β-N-acetylglucosaminidases were constitutively activated, even in the absence of (GlcNAc)2, but when both genes were deleted, no activity was detected. This confirms the model in which the sensor is held in the inactive state until the bacteria encounter chitin oligosaccharides, allowing efficient control of chitin catabolic genes.