The interaction of Escherichia coli host factor 1 with oligoadenylate [oligo(A)] was studied by fluorescence and filter retention techniques. The intrinsic fluorescence of the host factor is quenched by up to 60% by the addition of oligo(A). Fluorescence titrations at high protein concentrations (6 μM) give a saturation point of 14 A residues per host factor hexamer regardless of chain length or ionic strength. Nitrocellulose filter retention experiments at much lower concentrations (1 nM) indicate equimolar complexes form between (pA), (12 < l < 27) and host factor hexamers. The smallest number of contiguous A residues which allows the formation of all favorable protein-RNA contacts is 16 at both low and high salt concentrations. At 0.1 M NaCl, the molar association constants are in the range of 10 −10 -10 11 M −1 (15 < l < 27) and decrease only slightly with ionic strength, indicating a large nonionic component in the interaction. Cyclized (pA) l was shown to have a higher affinity for host factor than its linear counterparts when l is 18 or greater but a lower relative affinity when l is 15. This suggests that the binding site on the hexamer has a circular spatial orientation.
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