The mechanism of anaerobic regulation of synthesis of colicins E1, E2, E3, K and D was studied. It was found that anaerobiosis significantly increases expression of the genes for colicins E1, E2, E3, K, and D. Experiments with novobiocin (a DNA gyrase inhibitor) showed that colicin synthesis in minicells and derepressed colicin synthesis in cells are dramatically reduced by relaxation of DNA supercoiling. A good correlation was observed between the levels of colicin synthesis and plasmid DNA supercoiling and the degree of aeration of the cultures. Thus, the regulation of colicin gene expression in response to a change in aeration appears to be mediated by environmentally induced variations in DNA supercoiling.
The survivability of Escherichia coli K 12s cells has been studied after treatment with '251-labeled colicin El. It has been shown that for low amounts of adsorbed colicin the survivability follows single-hit kinetics. When the number of colicin molecules adsorbed exceeds approx. 50 per cell, deviation from single-hit kinetics occurs towards higher survivability. Colicin El adsorbed nonreceptorwise by the cell's surface has been shown to inhibit the lethal action of colicin El molecules adsorbed at specific receptors. This fact has been used in accounting for the elevated survivability of cells at high colicin doses. The functional significance of the phenomenon is discussed.
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