Catabolism of Raffinose,Sucrose, and Melibiose in Erwinia chrysanthemi 3937

Erwinia chrysanthemi (Dickeya dadantii) is a plant pathogenic bacterium that has a large capacity to degrade the plant cell wall polysaccharides. The present study reports the metabolic pathways used by E. chrysanthemi to assimilate the oligosaccharides sucrose and raffinose, which are particularly abundant plant sugars. E. chrysanthemi is able to use sucrose, raffinose, or melibiose as a sole carbon source for growth. The two gene clusters scrKYABR and rafRBA are necessary for their catabolism. The phenotypic analysis of scr and raf mutants revealed cross-links between the assimilation pathways of these oligosaccharides. Sucrose catabolism is mediated by the genes scrKYAB. While the raf cluster is sufficient to catabolize melibiose, it is incomplete for raffinose catabolism, which needs two additional steps that are provided by scrY and scrB. The scr and raf clusters are controlled by specific repressors, ScrR and RafR, respectively. Both clusters are controlled by the global activator of carbohydrate catabolism, the cyclic AMP receptor protein (CRP). E. chrysanthemi growth with lactose is possible only for mutants with a derepressed nonspecific lactose transport system, which was identified as RafB. RafR inactivation allows the bacteria to the assimilate the novel substrates lactose, lactulose, stachyose, and melibionic acid. The raf genes also are involved in the assimilation of α- and β-methyl-d-galactosides. Mutations in the raf or scr genes did not significantly affect E. chrysanthemi virulence. This could be explained by the large variety of carbon sources available in the plant tissue macerated by E. chrysanthemi.Pectinolytic erwiniae are enterobacteria that cause disease in a wide range of plants, including many crops of economic importance (23). The soft-rot symptom produced by Erwinia chrysanthemi (syn. Dickeya dadantii) results from the degradation of polysaccharides involved in the cohesion of the plant cell wall. The plant tissue maceration is concomitant with a large increase in the bacterial population (13). To ensure this multiplication, the bacteria assimilate various oligosaccharides released in the macerated tissue, which provide carbon and energy sources.E. chrysanthemi is known to use several carbon sources for growth, including sugars ranging from monosaccharides to polysaccharides. The completion of the E. chrysanthemi strain 3937 genome provides a genome-scale view into its potential catabolic capacities. A substantial part of the E. chrysanthemi genome is dedicated to genes involved in carbohydrate catabolism. In plant tissues, the most abundant soluble carbohydrates are the two oligosaccharides sucrose and raffinose (32). The trisaccharide raffinose α-d-Galp-(1→6)-α-d-Glcp-(1⇆2)β-d-Fruf] and the related disaccharides sucrose α-d-Glcp-(1⇆2)β-d-Fruf] and melibiose α-d-Galp-(1→6)-d-Glcp] are used as carbon sources for E. chrysanthemi growth. Previous studies suggested links between the transport of lactose and that of raffinose and melibiose (15). The E. chrysanthemi wild-type strain 3937 does not use lactose β-d-Galp-(1→4)-d-Glcp] as a carbon source for growth. This is due to the lack of a specific lactose transport system. However, spontaneous mutants able to assimilate lactose (designated Lac⁺) are easily obtained; they show a deregulation of the transport system LmrT, which is able to mediate lactose, melibiose, and raffinose transport (15). Despite our current knowledge of the strain 3937 genome sequence, no open reading frame (ORF) could be assigned to the lmrT gene, the identity of which remains unknown. We analyzed the E. chrysanthemi genome for the presence of potential genes involved in the catabolism of α-galactosides or α-glucosides. It contains a complete scrKYABR gene cluster that is involved in sucrose catabolism in various enterobacteria and a truncated rafRBA locus that is involved in raffinose catabolism. The growth with raffinose, despite the presence of an incomplete raf cluster, suggests that the missing functions are provided by other genes. Moreover, while E. chrysanthemi can catabolize melibiose, its genome does not contain homologues of the Escherichia coli melABR genes (30). Thus, to assimilate melibiose, E. chrysanthemi exploits other genes, which have yet to be identified. The present study mainly reports the role of the E. chrysanthemi gene clusters scr and raf in the catabolism of the oligosaccharides sucrose, raffinose, melibiose, and lactose. The importance of such catabolic pathways for bacterial multiplication in the plant tissues also was assessed during the infection process.