Role of the entC gene in enterobactin and menaquinone biosynthesis in Escherichia coli

Tn10 mutants of Escherichia coli MC4100 were screened for their inability to grow under iron deficiency and for their inability to grow under anaerobiosis in the presence of fumarate as an electron acceptor. A strain so obtained (E. coli PBB1) lacked the ability to convert chorismic acid to isochorismic acid. This shows that the gene (entC) encoding isochorismate synthase was mutated. E. coli PBB1 did not produce any detectable amounts of menaquinones (vitamin K2) or enterobactin. When supplemented with isochorismic acid this strain produced menaquinones, indicating that isochorismic acid is involved not only in enterobactin but also in menaquinone biosynthesis. The entC gene was isolated and was shown to be part of the enterobactin gene cluster: It was located on a DNA fragment (9 kb in length) which also carried the entA gene. The DNA fragment was identified by restriction site mapping and was compared to a previously published map of the enterobactin gene cluster. The entC gene on this fragment responds not only to conditions (iron deficiency) that stimulate enterobactin biosynthesis but also to anaerobiosis which results in increased isochorismic acid formation and increased menaquinone biosynthesis. We conclude that isochorismic acid, isochorismic synthase, and the gene (entC) encoding this enzyme are involved in catalytic events at a metabolic branch point from which both enterobactin and menaquinones originate.