Subcloning of the enterobactin biosynthetic gene entB: expression, purification, characterization, and substrate specificity of isochorismatase

The Escherichia coli entB gene, coding for the enterobactin biosynthetic enzyme isochorismatase, has been subcloned into the multicopy plasmid pKK223-3 under the control of the tac promoter. The resulting recombinant plasmid pFR1 expresses isochorismatase amounting to over 50% of the total cellular protein. The enzyme has been purified to homogeneity and a convenient assay developed. The enzyme has a Km for isochorismate of 14.7 microM and a turnover number of 600 min-1. By use of 1H NMR spectroscopy, the progress of the reaction was followed with the expected formation of 2,3-dihydro-2,3-dihydroxybenzoate product. Several substrate analogues were also utilized by the enzyme including chorismic acid, the immediate precursor to isochorismic acid in the enterobactin biosynthetic pathway.