THE PHOTOCHEMICAL INTERACTION OF DEOXYRIBONUCLEIC ACID AND PROTEIN IN VIVO AND ITS BIOLOGICAL IMPORTANCE*,†

Summary When bacteria are irradiated with u.v. light there is a dose dependent decrease in the amount of DNA that can subsequently be extracted free of protein with detergent. This appears to be due to the crosslinking of the DNA with protein and the precipitation of the linked DNA when the denatured proteins are precipitated in the procedure used for the isolation of the DNA. The type of linkage between the DNA and the protein is unknown except that it resists the sequential attack of 2% sodium lauryl sulfate and 0.5 M KCI or 55% CsCI. The main evidence that the loss of DNA in vivo is due to the crosslinking of DNA and protein is that the crosslinking of DNA and protein can be demonstrated in vitro . X-rays do not crosslink DNA and protein in vivo , but acridine orange and visible light cause the crosslinking of DNA and protein both in vivo and in vitro .
By pulse labeling the DNA of bacteria with tritated thymine it can be shown that newly synthesized DNA is most sensitive to crosslinking and that this sensitivity shows a cyclic response keyed to the generation time of the bacteria. Under conditions of thymine starvation where the intrinsic sensitivity of the cells to killing by u.v. is markedly increased, there is a parallel increase in the sensitivity of the DNA of these cells to be crosslinked to protein. The similarity in the time sequence of these two events strongly suggests that the crosslinking may play an important role in the loss of viability following u.v. irradiation.