Assimilation: Ketouronate Pathway

Abstract

The phytopathogenicity of Erwinia chrysanthemi seems to be related to its pectinolytic and cellulolytic activities. Polygalacturonate (PGA) can be degraded into galacturonate or 5-keto-4-deoxyuronate (DKI); 2-keto-3-deoxygluconate (KDG) is a common intermediate in the catabolism of these two compounds. The galacturonate catabolic pathway, first described in Erwinia carotovora, is similar to that existing in Escherichia coli. As described for the PGA metabolism of a phytopathogenic Pseudomonas DKI is isomerized to 2,5-diketo-3-deoxygluconate (DKII) and then reduced to KDG. The enzyme 2-keto-3-deoxygluconate: NAD-oxidoreductase (KDGDH) that converts DKII to KDG has been purified and its biochemical properties analyzed. Mutants impaired in pectin degradation were isolated by chemical mutagenesis and Mu d(Ap lac) insertion. A mutation in the kduD gene coding for KDGDH prevented the growth of the bacteria on polygalacturonate. In the kduD-lac fusion strains resulting from Mu d(Ap lac) insertion, β-galactosidase was induced by polygalacturonate, galacturonate and other intermediates of polygalacturonate catabolism. Galacturonate was inducer unless a mutation prevented its metabolizatin to KDG. Our results suggested that KDG and DKII are the true inducers of the synthesis of KDGDH and pectate lyase. Two kinds of regulatory mutants exhibiting constitutive β-galactosidase synthesis were isolated from the kduD-lac fusion strains. One type of mutant was also affected for pectate lyases synthesis.