Mutational analysis of predicted active site residues of an exoglucanase secreted by Xanthomonas oryzae pv. oryzae to determine their role in catalysis and in virulence on rice

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight disease in rice. As a part of its virulence repertoire, Xoo secretes a cell wall degrading enzyme Cellobiosidase (CbsA), which is a critical virulence factor and also a determinant of tissue specificity. CbsA protein is made up of an N-terminal catalytic domain and a C-terminal fibronectin type III domain. According to the CAZy classification, the catalytic domain of CbsA protein belongs to the glycosyl hydrolase-6 (GH6) family that performs acid-base catalysis. However, the identity of the catalytic acid and the catalytic base of CbsA is not known. Based on the available structural and biochemical data, we identified putative catalytic residues and probed them by site-directed mutagenesis. Intriguingly, the biochemical analysis showed that none of the mutations abolishes the catalytic activity of CbsA, an observation that is contrary to other GH6 family members. All the mutants exhibited altered enzymatic activity and caused significant virulence deficiency in Xoo emphasising the requirement of specific exoglucanase activity of wild-type CbsA for virulence on rice. Our study highlights the need for further studies and the detailed characterisation of bacterial exoglucanases.