Interaction between phage G13 and its oligosaccharide receptor studied by equilibrium dialysis

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Abstract
The reversible binding of phage G13, a ϕX174‐like single‐strand DNA phage, to a 3H‐labeled nonasaccharide from the lipopolysaccharide of its natural host Esacherichia coli C was studied with equilibrium dialysis. The binding constant (Ka) was determined to 1.3 × 107 M−1 in Scatchard and Lineweaver–Burk plots. Approximately one saccharide bound per G13 phage particle which suggests that only one of the 12 spikes in each G13 virion was engaged in the phage/receptor saccharide interaction. Equilibrium dialysis inhibition experiments with saccharides from lipopolysaccharides of an isogenic series of Salmonella typhimurium mutants showed that hepta‐ and pentasaccharides from two G13‐sensitive bacteria, i.e., with efficiencies of plating of 0.1–1.0 compared to E. coli C, were efficient inhibitors with Ka‐values ≥ 1.2 × 107 M−1. The octa‐ and hexasaccharides from two G13 resistant strains, with efficiency of plating ⩽ × 10−4, were either > 1000‐fold of > 15‐fold less efficient as inhibitors with Ka‐values ⩽8.8 − 105 M−1. The results show that phage G13 binds in a specific and reversible way to penta‐, hepta‐, and nonasaccharides from G13 sensitive bacteria with the specificity residing in the hexose and heptose region of the core lipopolysaccharide.