MUTATIONS IN PHOB, THE POSITIVE GENE ACTIVATOR OF THE PHO REGULON IN ESCHERICHIA-COLI, AFFECT THE CARBOHYDRATE PHENOTYPE ON MACCONKEY INDICATOR PLATES

Mutants defective in phoB, the positive gene activator of the Escheric hia coli pho regulon, exhibit aberrant behaviour on MacConkey indicato r plates. They appear pale in the presence of a fermentable carbon sou rce such as trehalose, maltose or glucose. The addition of at least 5 mM phosphate corrects this defect. Colonies of phoB+ strains turn red on MacConkey indicator plates and derepress the pho regulon when the c ells are able to ferment the carbon source. In contrast, the inability to ferment the carbon source maintains the pho regulon in the repress ed state.