Peculiarities of the secondary structure of bacteriophage DNA in situ: VI. The reaction of S D phage with glyoxal

Abstract

Glyoxal-induced modification of DNA in S D phage and the effect of Mg 2+ and Na + ions on this process have been studied. During the early stages of the reaction (60 min, 40 °C, 3% glyoxal), in the phage suspension devoid of Mg 2+ there develops a hypochromic effect, the value of which at 260 nm is 7.0 ± 0.5%. At the same time, the CD spectrum of the intraphage DNA changes towards that characteristic of free native DNA; the value of Δ A 283 increases from 1.0 × 10 −4 to 1.4 × 10 −4, and the transition point shifts from 270 to 263 nm. The data obtained are interpreted as indicating a glyoxal-induced conformational transition in the intraphage DNA, which resembles that induced by formaldehyde (T. I. Tikchonenko et al. (1974) Arch. Biochem. Biophys. 160, 1–13). The presence of Mg 2+, which does not affect the rate of modification of free native DNA, strongly affects the reaction of intraphage DNA with glyoxal: It prevents the change of CD spectra, it reverses the absorption effect from a hypo- to a hyperchromic shift, which is due to the modification of cytosine and adenine nuclei, and it allows 15–25% of the guanine bases to be modified as determined by chemical analysis. This last result is in good agreement with previous data which suggested that about 20% of the cytosines in intraphage DNA are out of the double helix. The most probable explanation for the effect of Mg 2+ is that divalent cations stabilize specific noncovalent interactions of the bases with the proteins in the phage corpuscle, thereby preventing the conformational transition from taking place.