Molecular cloning, nucleotide sequence, and marker exchange mutagenesis of the exo-poly-alpha-D-galacturonosidase-encoding pehX gene of Erwinia chrysanthemi EC16.

The pehX gene encoding extracellular exo-poly-alpha-D-galacturonosidase (exoPG; EC 3.2.1.82) was isolated from a genomic library of the pectate lyase-deficient Erwinia chrysanthemi mutant UM1005 (a Nalr Kanr delta pelABCE derivative of EC16) by immunoscreening 2,800 Escherichia coli HB101 transformants with an antibody against exoPG protein. The cloned pehX gene was expressed highly from its own promoter in E. coli, and most of the enzyme was localized in the periplasm. The nucleotide sequence of pehX revealed the presence of an amino-terminal signal peptide and an open reading frame encoding a preprotein of 64,608 daltons. The cloned pehX gene was insertionally inactivated with TnphoA and used to mutate the chromosomal pehX gene of E. chrysanthemi AC4150 (Nalr) and CUCPB5006 (Nalr Kans delta pelABCE) by marker exchange mutagenesis. Analysis of the resulting mutants, CUCPB5008 (Pel+ Peh-) and CUCPB5009 (Pel- Peh-), indicated that exoPG can contribute significantly to bacterial utilization of polygalacturonate and the induction of pectate lyase in the presence of extracellular pectic polymers. CUCPB5009 retained a slight ability to pit polygalacturonate semisolid agar and macerated chrysanthemum pith tissues when large numbers of bacteria were inoculated.