Figure 3.

Key steps in the chitinolytic cascade. Functional assignment of A. salmonicida CDSs are derived from bioinformatical analysis, and the abbreviated gene IDs are indicated at each step. Putative non-functional A. salmonicida products are boxed. Initially, a methyl chemotaxis protein (VSAL_II0134) and a chitin binding protein (VSAL_I2601) are involved in sensing and attachment to chitin respectively. Extracellular chitinases (VSAL_I0763/I0902, VSAL_I1414, VSAL_I1942, VSAL_I0757 and VSAL_II0217) partly break down chitin. Chitin oligomers (GlcNAc)_n are translocated across the outer membrane by chitoporin (VSAL_I2352), while the transport of monomers and dimers (GlcNAc)_1,2 is mediated by unspecific porins. In the periplasm, the chitin oligomers are further degraded to yield (GlcNAc)_1,2 by chitodextrinase (VSAL_I1108), N-acetylglucosamidase (VSAL_I2583 and VSAL_II0052) and N-acetylhexosamidase (VSAL_II0575). (GlcNAc)₁ are transported across the inner membrane by a permease (VSAL_I0831), while the transport of (GlcNAc)₂ are mediated by an ABC-transporter (VSAL_I2577- I2580). Deacetylated monomers (GlcN) are transported into the cytosol by a PTS transporter (VSAL_I0703- I0708). Cytoplasmic enzymes (VSAL_I2581- I2582, VSAL_I2584- I2585, VSAL_I0830 and VSAL_I2812) convert the transport products into fructose-6-P, acetate and ammonia. In the absence of chitin, the perisplasmic chitin oligosaccharide binding protein CBP (VSAL_I2576) binds to the chitin sensor/kinase ChiS (VSAL_I2575) and represses transcription of chitinolytic genes. In presence of chitin, the complex dissociates as CBP binds to the chitin polysaccharides and chitinolytic genes are expressed.