The bacterium E. coli contains two major quinones, ubiquinone and menaquinone or vitamin K2. In an early paper Cox and Gibson, having established appropriate methods for the extraction and identification of the quinones, used radio-labelled shikimate to demonstrate that both ubiquinone and menaquinone were products of the shikimic acid pathway. In the same experiments, by using excess unlabelled 4-hydroxybenzoic acid, they showed that this compound was an intermediate in the synthesis of ubiquinone but not in the synthesis of menaquinone (24, 30). Subsequently, by using cell-free extracts of a wild-type strain grown in the presence of the three aromatic amino acids, they were able to demonstrate the conversion of radio-labelled chorismic acid into both ubiquinone and menaquinone. As was reported later, they had also observed that certain multiple aromatic auxotrophs with a complete block in the common pathway could grow on a glucose mineral salts medium supplemented with the three aromatic amino acids, 2,3-dihydroxybenzoic acid and p-aminobenzoate. Under these conditions the cells failed to synthesise either ubiquinone or menaquinone