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Journal of Bacteriology, March 1999, p. 1652-1663, Vol. 181, No. 5
Laboratoire de Génétique
Moléculaire des Microorganismes, UMR-CNRS 5577, INSA, F-69621
Villeurbanne Cedex, France,1 and Section
Molecular Genetics of Industrial Microorganisms, 6703 HA Wageningen,
The Netherlands2
Received 22 September 1998/Accepted 21 December 1998
Erwinia chrysanthemi 3937 secretes several pectinolytic
enzymes, among which eight isoenzymes of pectate lyases with an
endo-cleaving mode (PelA, PelB, PelC, PelD, PelE, PelI, PelL, and PelZ)
have been identified. Two exo-cleaving enzymes, the
exopolygalacturonate lyase, PelX, and an
exo-poly-
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Characterization of the Exopolygalacturonate Lyase
PelX of Erwinia chrysanthemi 3937
-D-galacturonosidase, PehX, have been
previously identified in other E. chrysanthemi strains.
Using a genomic bank of a 3937 mutant with the major pel
genes deleted, we cloned a pectinase gene identified as
pelX, encoding the exopolygalacturonate lyase. The deduced
amino acid sequence of the 3937 PelX is very similar to the PelX of
another E. chrysanthemi strain, EC16, except in the 43 C-terminal amino acids. PelX also has homology to the endo-pectate
lyase PelL of E. chrysanthemi but has a N-terminal extension of 324 residues. The transcription of pelX,
analyzed by gene fusions, is dependent on several environmental
conditions. It is induced by pectic catabolic products and affected by
growth phase, oxygen limitation, nitrogen starvation, and catabolite repression. Regulation of pelX expression is dependent on
the KdgR repressor, which controls almost all the steps of pectin catabolism, and on the global activator of sugar catabolism, cyclic AMP
receptor protein. In contrast, PecS and PecT, two repressors of the
transcription of most pectate lyase genes, are not involved in
pelX expression. The pelX mutant displayed
reduced pathogenicity on chicory leaves, but its virulence on potato
tubers or Saintpaulia ionantha plants did not appear to be
affected. The purified PelX protein has no maceration activity on plant
tissues. Tetragalacturonate is the best substrate of PelX, but PelX
also has good activity on longer oligomers. Therefore, the estimated
number of binding subsites for PelX is 4, extending from subsites
2
to +2. PelX and PehX were shown to be localized in the periplasm of
E. chrysanthemi 3937. PelX catalyzed the formation of
unsaturated digalacturonates by attack from the reducing end of the
substrate, while PehX released digalacturonates by attack from the
nonreducing end of the substrate. Thus, the two types of exo-degrading
enzymes appeared complementary in the degradation of pectic polymers,
since they act on both extremities of the polymeric chain.
*
Corresponding author. Mailing address:
Laboratoire de Génétique Moléculaire des
Microorganismes, UMR-CNRS 5577, INSA, Bat 406, 20 Av. A. Einstein,
F-69621 Villeurbanne Cedex, France. Phone: (33) 472-43-80-88. Fax: (33)
472-43-87-14. E-mail: cotte{at}insa.insa-lyon.fr.
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