The net hydration of phage lambda

The banding density of phage lambda varies with the activity of water when the phage particles are banded in a series of different cesium salts. The results are comparable to those Hearst and Vinograd for free DNA. Lambda phage ghosts show less net hydration than the phage particles and band in a fairly narrow range of densities in these cesium salts. The phage banding density may be predicted to a first approximation by a simple additive approximation: the total net hydration of the phage is approximately equal to the net hydrations of free λ DNA λ hosts, all measured at the same water activity. The simple additive approximation is not adequate, however, to explain the banding density differences between a deletion mutant and phage lambda in the different cesium salts. The density differences evidently are sensitive to second‐order effects: they apparently are affected by a restriction of DNA hydration inside the phage head, which depends both on water activity and on DNA length (or free volume inside the phage head). This becomes a striking effect in Cs 2 SO 4 solutions where the net DNA hydration is large. Changing the phage banding density by substituting 5‐bromouracil for thymine, which increases the DNA mass while leaving the DNA volume relatively unchanged, gives results consistent with a restriction of the net DNA hydration that depends on the DNA volume. Data on the sedimentation velocity behavior that λ and λ b 2 in diferrent salts are presented and discussed. It appears possible to estimate the size of a DNA deletion from the phage sedimentation coefficient.