REGULATION OF PECTINOLYSIS IN ERWINIA CHRYSANTHEMI

Abstract

Erwinia chrysanthemi is an enterobacterium that causes various plant diseases. Its pathogenicity results from the secretion of pectinolytic enzymes responsible for the disorganization of the plant cell wall. The E. chrysanthemi strain 3937 produces two pectin methylesterases, at least seven pectate lyases, a polygalacturonase, and a pectin lyase. The extracellular degradation of the pectin leads to the formation of oligogalacturonides that are catabolized through an intracellular pathway. The pectinase genes are expressed from independent cistrons, and their transcription is favored by environmental conditions such as presence of pectin and plant extracts, stationary growth phase, low temperature, oxygen or iron limitation, and so on. Moreover, transcription of the pectin lyase gene responds to DNA-damaging agents. The differential expressions of individual pectinase genes presumably reflect their role during plant infection. The regulation of pel genes requires several regulatory systems, including the KdgR repressor, which mediates the induction of all the pectinolysis genes in the presence of pectin catabolites. KdgR also controls the genes necessary for pectinase secretion and other pectin-inducible genes not yet characterized. PecS, a cytoplasmic protein homologous to other transcriptional regulators, can bind in vitro to the regulatory regions of pectinase and cellulase genes. The PecT protein, a member of the LysR family of transcriptional regulators, represses the expression of some pectinase genes and also affects other metabolic pathways of the bacteria. Other proteins involved in global regulations, such as CRP or HNS, can bind to the regulatory regions of the pectinase genes and affect their transcription.

The Oligogalacturonate-specific Porin KdgM of Erwinia chrysanthemi Belongs to a New Porin Family

“…Induction in the presence of pectin derivatives mainly results from the interaction of the KdgR repressor with intracellular pectic catabolites such as and 5-keto-4-deoxyuronate or 2-keto-3-deoxygluconate (2). A 10-fold increase in the fusion expression in a kdgR background (Table III) confirmed that the kdgM expression is controlled by the main regulator of genes involved in pectin degradation, KdgR (2,41,47). However, the expression of the fusion in a kdgR mutant is still induced by GA. Transcriptional regulation of genes involved in pectin and GA degradation pathways is independent.…”

“…E. chrysanthemi possesses aspecific porins similar to the OmpF and OmpC porins of E. coli, which are responsible for membrane permeability for molecules of Ͻ600 Da (14). Such porins could be responsible for GA 2 and GA 3 uptake (GA 3 is 546 Da) but not for the uptake of larger GA n . When grown in the presence of GA n (Fig.…”

“…4), encoding an esterase. 2 The presence of a signal sequence, suggested by a N-terminal hydrophobic region of 20 amino acids followed by the sequence Ala-Leu-Ala, which is a potential cleavage site for the LepB peptidase (5), was confirmed by the N-terminal sequencing of the protein purified from E. chrysanthemi membranes. KdgM C-terminal primary structure, with its Phe terminal residue, is characteristic of outer membrane proteins (42).…”

“…Pectin catabolism involves a variety of pectinases, including esterases and depolymerases ( Fig. 1), among which endo-pectate lyases play a crucial role in the soft-rot disease (2,3). Pectinases are secreted in the extracellular medium by a type II secretion pathway, the Out system (4,5).…”

“…GA 3 and GA 4 are good substrates for the cytoplasmic pectate lyase PelW, and GA 2 is the preferential substrate for the cytoplasmic oligogalacturonate lyase Ogl (11). After being internalized into the cytoplasm, GA n are catabolized by PelW and Ogl into monomers of GA and 5-keto-4-deoxyuronate, which are next degraded into 2-keto-3-deoxygluconate and then into pyruvate and 3-phosphoglyceraldehyde, products that finally enter the general cellular metabolism (2,11). Recently, two transport systems allowing GA n translocation through the inner membrane, TogMNAB and TogT, have been characterized (12,13).…”