A family 3 glycosyl hydrolase of Dickeya dadantii 3937 is involved in the cleavage of aromatic glucosides

Abstract

Dickeya dadantii is a phytopathogenic bacterium secreting a large array of plant-cell-walldegrading enzymes that participate in the infection and maceration of the host plant tissue. Sequencing of the D. dadantii 3937 genome predicted several genes encoding potential glycosidases. One of these genes, bgxA, encodes a protein classified in family 3 of glycosyl hydrolases. Inactivation of bgxA and the use of a gene fusion revealed that this gene is not essential for D. dadantii pathogenicity but that it is expressed during plant infection. The bgxA expression is induced in the presence of glycosidic or non-glycosidic aromatic compounds, notably ferulic acid, cinnamic acid, vanillic acid and salicin. The BgxA enzyme has a principal b-Dglucopyranosidase activity and a secondary b-D-xylopyranosidase activity (ratio 70 : 1). This enzyme activity is inhibited by different aromatic glycosides or phenolic compounds, in particular salicin, arbutin, ferulic acid and vanillic acid. Together, the induction effects and the enzyme inhibition suggest that BgxA is mostly involved in the cleavage of aromatic b-glucosides. There is evidence of functional redundancy in the D. dadantii b-glucoside assimilation pathway. In contrast to other b-glucoside assimilation systems, involving cytoplasmic phospho-b-glucosidases, the cleavage of aromatic glucosides in the periplasmic space by BgxA may avoid the release of a toxic phenolic aglycone into the cytoplasm while still allowing for catabolism of the glucose moiety. INTRODUCTIONb-Glucosidases (b-D-glucopyranoside glucohydrolases, E.C.3.2.1.21) are a heterogeneous group of enzymes, which hydrolyse glycosidic bonds to release non-reducing terminal glucosyl residues from glycosides and oligosaccharides. These enzymes catalyse the hydrolysis of cellobiose and various b-glucosides, the aglycone of which can be an aromatic compound (Ketudat Cairns & Esen, 2010). bGlucosidases are universally found in all domains of living organisms, archaea, eubacteria and eukaryotes. Most microbial b-glucosidases have been investigated in the soil and plant microflora and they have been considered as the ultimate enzymic step in the biological conversion of cellulose into glucose. However, other b-glucosidases preferentially hydrolyse aryl b-glucosides, such as arbutin and salicin (Faure et al., 1999). b-Glucosidases have been the focus of several studies because of their roles in a variety of biological processes, namely, the conversion of cellulose to glucose, the release of aromatic compounds from flavourless glucosidic precursors, the detoxification of cyanogenic glucosides, and the synthesis of useful b-glucosides (Bhatia et al., 2002;Ismail & Hayes, 2005;Ketudat-Cairns & Esen, 2010). The classic enzyme classification (EC) system groups together glycoside hydrolases according to their substrate specificity, for example, the bglucosidases all have the designation EC3.2.1.21. An alternative classification system has been developed for glycoside hydrolases, based on amino acid sequence and structural simila...

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