Internal motion of DNA in bacteriophages

We have investigated internal motion of DNA in bacteriophages by measuring fluorescence anisotropy decays of intercalated ethidium. The results showed large suppression of the internal motion of the inner DNA; the interhelix interaction of the DNA in the phage head is considered to enhance the effective viscosity of the DNA rod and to restrict the angle of the internal motion. Considering that the observed internal motion arises mainly from torsional motion of the DNA, we have calculated the movable angles of the torsional motion (the standard deviation of the torsional motion) of the DNA in the phage heads. The magnitude of the calculated movable angles indicates the extent of suppression of the DNA movement in the phage head; in λ wild type phage, the DNA is packed most rigidly in the head and the motion is found to be restricted most severely. In a deletion mutant of λ phage, whose inner DNA content is deficient by 17.6%, steric hindrance from the interhelix DNA interaction is decreased, and the DNA can move easily. In T4 wild type phage, although the extent of condensation of the inner DNA is the same as that in λ wild type phage, the DNA was fairly mobile. The presence of glucosylated hydroxymethylcytosine is suggested to influence the rigidity of the inner DNA or packaging mode of the DNA in the T4 head.