Clustering of isochorismate synthase genes menF and entC and channeling of isochorismate in Escherichia coli.

There are two isochorismate synthase genes entC and menF in Escherichia coli. They encode enzymes (isochorismate synthase, EC 5.4.99.6) which reversibly synthesize isochorismic acid from chorismic acid. The genes share a 24.2% identity but are differently regulated. Activity of the MenF isochorismate synthase is significantly increased under anaerobic conditions whereas the activity of the EntC isochorismate synthase is greatly stimulated during growth in an iron deficient medium. Isochorismic acid synthesized by EntC is mainly channeled into enterobactin synthesis whereas isochorismic acid synthesized by MenF is mainly channeled into menaquinone synthesis. When menF or entC were separately placed onto overexpression plasmids and the plasmids introduced into a menF(-)/entC(-) double mutant in two separate experiments, the isochorismate formed was fed into both, the menaquinone and the enterobactin pathway. Moreover, in spite of a high isochorismate synthase activity menaquinone and enterobactin formation were not fully restored, indicating that isochorismate was lost by diffusion. Thus, under these conditions channeling was not observed. We conclude that in E. coli the chromosomal position of both menF and entC in their respective clusters is a prerequisite for channeling of isochorismate in both pathways.