[DNA structure in the particles of 5 bacteriophages from the data of circular dichroism]

DNA optical activities in situ were studied in the particles of five medium sized bacteriophages (SB1, F15; IRA, SD and T7). Delta epsilon in the CD spectrum of intraphage DNA is not shown to correlate with the sizes of phage heads or with the light scattering characteristics of phage suspension. Bacteriophage SB1, studied for the first time, has the amplitude of CD spectrum in the 260-300 nm region higher, than the CD spectrum of its free phage DNA. CD magnitude in the 260-300 nm region is different for varying phages while the red shift of the positive band in the CD spectrum takes place for all phages studied. The dense packing of DNA is suggested to be a common factor for the red shift being observed for all phages. The different delta epsilon in the 260-300 nm region might reflect the different nature in changes of helical DNA geometry inside phage particles as compared with the changes in solutions. The increase in melting temperatures for intraphage DNA as compared with the temperature for free DNA was not shown to correlate with the CD spectrum difference of intraphage DNAs. This property of intraphage DNAs is supposed to be connected with the dense packing of DNA in bacteriophage deoxyribonucleoprotein.