Impaired induction and self-catabolite repression of extracellular pectate lyase in Erwinia chrysanthemi mutants deficient in oligogalacturonide lyase

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The pectate lyase (PL; EC 4.2.2.2) secreted by the plant pathogen Erwinia chrysanthemi is induced and catabolite repressed by different concentrations of its own product, digalacturonic acid 4,5-unsaturated at the nonreducing end [u(GalUA)2]. Both activities of u(GalUA)2 depend on its cleavage by oligogalacturonide lyase (OGL; EC 4.2.2.6). This intracellular enzyme converts u(GalUA)2 to the deoxyketuronic acid 4-deoxy-L-threo-5-hexosulose uronic acid, which is then isomerized to 3-deoxy-D-glycero-2,5-hexodiulosonic acid. An OGL-deficient mutant unable to grow on u(GalUA)2 was poorly induced by u(GalUA)2 or by D-galacturonan but produced wild-type levels of PL when supplied with 3-deoxy-D-glycero-2,5-hexodiulosonic acid. PL synthesis in the mutant could also be stimulated by 4,5-unsaturated trigalacturonic acid, from which deoxyketuronic acid is released by another intracellular enzyme. An OGL-deficient mutant that grew slowly on u(GalUA)2 in comparison with the wild-type parent was hyperinduced by u(GalUA)2 unless catabolite repression was relieved by cyclic AMP or imposed by logarithmic growth on glycerol. PL synthesis is also stimulated by saturated digalacturonic acid, which is released from D-galacturonan by another extracellular enzyme, exo-poly-α-D-galacturonosidase (EC 3.2.1.82). Because these dimers stimulate PL synthesis at concentrations (wt/vol) 1/1000th of the concentration required by D-galacturonan, and because an OGL-deficient mutant uninducible by dimers was also uninducible by D-galacturonan, we postulate that PL induction by pectic polymers entails extracellular formation of dimers and subsequent intracellular conversion to deoxyketuronic acids, the apparent inducers of PL.

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