The transfer of the restriction nuclease EcoRI from nonspecific association with poly(dI-dC) to its recognition sequence is accompanied by the release of 110 waters that is independent of the solute used to set water activity for a wide range of solute sizes and chemical identities. As was first suggested by our previous measurements on the competitive binding of E. coli galactose operon repressor and is now confirmed by the more extensive measurements with EcoRI, the effect of solutes on the transfer reaction of protein from one DNA sequence to another is an ideally osmotic one. There is a well defined volume of water at the protein - DNA interface that is sterically sequestered from solute in the bulk solution. Water release can now be regarded as both a thermodynamic and a structural parameter, linking energetics and closeness of contact. We can now investigate in detail the relationship between water release and association energies using Cro protein and a series of DNA recognition sequences that span a range of binding constants. The measurement of forces between molecules in condensed arrays show a dominating contribution from water structuring. The previous measurements on the binding of gal repressor to different operator sequences show that a link between binding strength and structured water release does not exist for association reactions in dilute solution. Changes in the structure and flexibility of Acanthamoeba myosin II minifilaments with ATP or ADP binding have been further characterized by electric birefringence. Minifilaments with bound ATP or ADP are about 20 fold more flexible than without nucleotide. This change in flexibility seems directly relevant to the force generating Step in actin-myosin complexes. Comparison of signal amplitudes for native myosin II minifilaments and minifilaments constructed from truncated myosin (missing the S1 heads) indicates that flexing occurs at the LMM-HMMjunction. Changes in the fist component relaxation kinetics indicates that nucleotide binding affects the structure of the S1-S2 junction or of the S1 heads as well as the flexibility of the LMM-HMM junction. The nature of the structural coupling of these two physically well separated regions is still unknown, but may involve the light chains.
