Peculiarities of the secondary structure of bacteriophage DNA in situ. V. Change in DNA conformation inside the phages under the influence of formaldehyde

Abstract

We have studied the changes in uv absorption and ORD in early and late stages of the reaction of formaldehyde with DNA inside Sd and T2 phages. The 20-to 25-min incubation of phages with formaldehyde at temperatures of about 40°C does not disrupt the phage particles but nevertheless causes the development of a typical hypochromic effect reaching 10±1% at 260 nm and similar to the hypochromism that develops during disruption of the phage in a medium without HCHO. The complete additivity is observed between the hypochromism at 260 nm arising as a result of incubation with HCHO and the hypochromic effect produced by disruption of the phage. The development of hypochromism under these conditions is accompanied by restoration of the positive peak of rotation with a maximum at 289 nm, characteristic of native DNA in solution and greatly reduced in intraphage DNA. The observed effects are interpreted as a conformational transition—a restoration of the Watson-Crick type of structure in the part of DNA whose secondary structure was changed in situ due to interaction with protein. All these optical changes of intraphage DNA under the influence of formaldehyde are reversible. The removal of HCHO leads to restoration of the anomalous optical properties of intraphage DNA, including deficit of hypochromism and a sharp decrease in positive rotation in the region of 280–290 nm. The rate constants, energy of activation, and entropy of activation for the reactions of formaldehyde with a mixture of bases, with free native and intraphage DNA were calculated.