Characterization of pectin methylesterase B, an outer membrane lipoprotein of Erwinia chrysanthemi 3937.

The secretion of extracellular pectinases, among which there are least six isoenzymes of pectate lyase and one pectin methylesterase, allows the phytopathogenic bacterium Erwinia chrysanthemi to degrade pectin. A gene coding for a novel pectin methylesterase has been cloned from an E. chrysanthemi strain 3937 gene library. This gene, pemB, codes for a 433-amino-acid protein. The PemB N-terminal region has the characteristics of lipoprotein signal sequences. We have shown that the PemB precursor is processed and that palmitate is incorporated into the mature protein. The PemB lipoprotein is not released into the extracellular medium and is localized in the outer membrane. The PemB sequence presents homology with other pectin methylesterases from bacterial and plant origin. pemB-like proteins were detected in four other E. chrysanthemi strains but not in Erwinia carotovora strains. PemB was overproduced in Escherichia coli and purified to homogeneity. PemB activity is strongly increased by non-ionic detergents. The enzyme is more active on methylated oligogalacturonides than on pectin, and it is necessary for the growth of the bacteria on oligomeric substrates. PemB is more probably involved in the degradation of methylated oligogalacturonides present in the periplasm of the bacteria, rather than in a direct action on extracellular pectin. pemB expression is inducible in the presence of pectin and is controlled by the negative regulator KdgR.