| Abstract: | Proteins in the outer membrane of gram-negative bacteria serve as general porins or as receptors for specific nutrient transport systems. Many of these proteins are also used as receptors initiating the processes of colicin or phage binding and uptake. The functional activities of several outer membrane proteins in Escherichia coli K-12 were followed after cessation or repression of their synthesis. Cessation of receptor synthesis was accomplished with a thermolabile suppressor activity acting on amber mutations in btuB (encoding the receptor for vitamin B(12), the E colicins, and phage BF23) and in fepA (encoding the receptor for ferric enterochelin and colicins B and D). After cessation of receptor synthesis, cells rapidly became insensitive to the colicins using that receptor. Treatment with spectinomycin or rifampin blocked appearance of insensitive cells and even increased susceptibility to colicin E1. Insensitivity to phage BF23 appeared only after a lag of about one division time, and the receptors remained functional for B(12) uptake throughout. Therefore, possession of receptor is insufficient for colicin sensitivity, and some interaction of receptor with subsequent uptake components is indicated. Another example of physiological alteration of colicin sensitivity is the protection against many of the tonB-dependent colicins afforded by provision of iron-supplying siderophores. The rate of acquisition of this nonspecific protection was found to be consistent with the repression of receptor synthesis, rather than through direct and immediate effects on the tonB product or other components of colicin uptake or action. |
