Irreversible binding of phage phi X174 to cell-bound lipopolysaccharide receptors and release of virus-receptor complexes.

At 37 degrees C, binding of phi X174 to the lipopolysaccharide receptors in the outer membrane of Escherichia coli C is followed by an irreversible ejection of its DNA. DNA ejection marks the beginning of the eclipse period in the infection cycle. Binding data with a phi X mutant Fcs70 at 15 degrees C, where the DNA ejection, or eclipse, rate is essentially zero, do not follow the law of mass action. This rules out a simple mechanism of reversible binding followed by irreversible DNA ejection. A more complex reaction model was devised to fit the data [Incardona, N. L. (1983) J. Theor. Biol. 105, 631-645]. It takes into account the fact that lipopolysaccharide-containing outer membrane fragments are continually released from infected E. coli cells, some of which have phi X bound to them. In this paper the model is shown to fit the binding data for wild-type virus at 15 degrees C and to account for the nonlinearity observed at 37 degrees C in the pseudo-first-order binding kinetics and first-order eclipse kinetics for both mutant and wild-type virus. This leads to the conclusion that phi X174 binding to cell-bound receptors is irreversible but binding to released receptors is reversible. The release of virus-receptor complexes from infected cells and the dissociation of these complexes were confirmed by electron microscopy. We propose that initially a single phi X174 vertex interacts reversibly with E. coli lipopolysaccharide but dissociation from the cell is prevented by the subsequent interaction of additional vertices with adjacent receptor molecules.