FIG 6 .

Galactan metabolism in B. subtilis and other bacteria. (A) Proposed pathway in B. subtilis for utilization of naturally occurring galactan. Galactan is first broken down to galactotriose by GanB outside the cell. Galactotriose is then taken up by the permease composed of CycB-GanP-GanQ and further broken down to galactose by GanA. Galactose is eventually converted to glucose by the enzymes in the Leloir pathway encoded by the gal genes or becomes a sugar substrate of EPS during biofilm formation. (B) Genetic arrangements of the putative gene clusters for utilization of galactan in various bacteria. In two plant-associated Bacillus species, B. licheniformis and B. amyloliquefaciens, as well as in a gut-associated bacterium, Lactobacillus acidophilus, the homologous (to cycB-ganPQAB in B. subtilis) genes whose protein products are predicted to function in the breakdown of galactan to galactose are not only clustered but also grouped with the gal genes whose protein products are involved in further metabolism of galactose. Conservation of this genetic arrangement of the above-described genes in various bacteria implies that these bacteria adapted to utilize host-derived sugar polymers. In L. acidophilus, the gene cluster contains three ganA paralogs (light green) and two unknown phage-like genes (gray). In B. amyloliquefaciens, the two permease-like genes (blue) encode a putative PtII sugar uptake system, whereas in L. acidophilus, a single lacS gene (blue) encodes a putative sugar permease.