Peculiarities of the secondary structure of bacteriophage DNA in situ: II. Reaction with formaldehyde

The state of the amino groups of the DNA bases inside phage particles was studied with formaldehyde in experiments of three kinds. In the first set of experiments, phage-dodecyl sulphate complexes were disintegrated in the presence of formaldehyde; and the effect of the latter on the conformational transition of the DNA during its release into the medium was determined. In the second set of experiments, the melting of DNA inside the phage in the presence of various concentrations of formaldehyde was investigated under conditions preventing disruption of the phage particle. Finally, the effect of methylene cross-links on the conformational transition of DNA as well as on its melting point was elucidated in experiments with “fixed” (“formalinized”) phage. A small but definite number of free amino groups, most probably forming small regions with the disordered secondary structure, are thought to be present in DNA inside the phage particle. The rest of the amino groups of the DNA bases inside the phage possess an increased stability to formaldehyde. DNA inside the phage can be characterized according to the melting curves by a considerable loss of co-operativeness in interaction of the bases and by extended phase transition during thermal denaturation.