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

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Abstract
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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