Characterization and overexpression of the pem gene encoding pectin methylesterase of Erwinia chrysanthemi strain 3937.

The pem gene encoding the pectin methylesterase (PME) of Erwinia chrysanthemi strain 3937 was subcloned and its nucleotide sequence determined. The gene contains an open reading frame of 1098 bp and codes for a protein of 366 amino acids (aa). The mature 37-kDa form of the protein is 342 aa long and has a calculated isoelectric point of 9.64. A plasmid was constructed to overproduce PME: a DNA fragment carrying pem was amplified by the polymerase chain reaction and cloned downstream from the pL promoter of the lambda phage, in a high-copy-number plasmid. In an Escherichia coli strain transformed with this plasmid, an increase in PME production of more than 60-fold was obtained, compared with the wild-type Er. chrysanthemi strain. PME represents about 5% of the total protein content of the cells. Comparison of this PME sequence with six PMEs from prokaryotic or eukaryotic organisms showed six highly conserved segments whose possible role in enzyme activity are discussed.