Chemical modification of the lactose carrier of Escherichia coli by plumbagin, phenylarsinoxide or diethylpyrocarbonate affects the binding of galactoside

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Abstract
The effects of chemical modification of the lactose carrier of E. coli on galactoside binding (in overproducing strains) and on transport was examined. The modifying reagents diethylpyrocarbonate and rose bengal and the thiol reagents phenylarsine oxide and plumbagin can completely inhibit the binding of the substrate p-nitrophenyl .alpha.-D-galactopyranoside to the carrier. If care is taken to inhibit galactoside binding only partially, the loss of transport is found to parallel the loss of binding sites. The modified carrier molecules are completely inactive, while the remaining active carrier molecules evince normal transport and binding parameters. The binding of galactoside protects the carrier partially against these forms of chemical modification. In view of these observations, the results of previous chemical modification studies must be re-interpreted. The results stress the utility of studying substrate binding, the first step in the transport cycle.