Genetic and biochemical characterization of hemicellulose polysaccharides utilization in Bacillus subtilis

Plant biomass is the major source of available carbohydrates in Nature. The walls of plant cells are mainly composed by homo- and heteropolysaccharides, usually found intertwined with each other and making a highly complex and heterogeneous structure, which displays a strong resistance to biological degradation. Still, many microorganisms are able to breakdown this complex structure and use the available sugars as carbon and energy source. Several saprophytic bacteria, including Bacillus subtilis, release a high number of saccharolytic enzymes capable of cleaving most polysaccharides in plant cell walls, and are then able to transport and metabolize the resulting mono- and oligosaccharides. Thus, transport is a key step in sugar utilization. Here, we characterize four ABC-type transporters involved in the uptake of arabino- and galactooligosaccharides, and galacturonic acid oligomers and/or rhamnose-galacturonic acid disaccharides. We show that MsmX is the sole ATPase responsible for energizing these four ABC importers, thus playing a key role in pectin mobilization by B. subtilis. AraN, the solute-binding protein (SBP) from the ABC importer, AraNPQ, is shown by in vivo and in vitro analyses to bind arabinooligosaccharides. However, AraN displays a significantly lower affinity than that previously reported for other SBPs. In addition, AraN also binds cello-, xylo- and maltooligosaccharides with similar affinity but is shown to be non-essential for growth in the presence of these oligosaccharides. We speculate that binding of these sugars may prevent arabinooligosaccharides uptake when more favorable carbon sources, such as glucose-rich cello- and maltodextrins, are available. The mechanisms of expression of abn2, a gene encoding an endo-arabinanase, were investigated and we provide evidence that this AraR-independent gene is temporally regulated. The transition state regulator AbrB and the master sporulation regulator Spo0A are implicated in the regulation of abn2. Finally, we demonstrate that a metallic cluster close to the catalytic site of Abn2 is essential for its arabinanase activity