Comparison of the in vitro action of ethidium chloride on animal viruses with that of other photodyes.

Four viruses were irreversibly photosensitized by 4 × IO-SM-ethidium chloride but not by more dilute solutions. Photosensitization was greatest with vaccinia, pseudorabies, herpes tamarinus and reoviruses, intermediate with western equine encephalitis virus, and least with fowl plague and Newcastle disease viruses. Pseudorabies virus was also inactivated by ethidium chloride at 37 ° in the dark. The order of susceptibility of the viruses to other dyes varied according to the concentration of dye. Native thymus DNA, but not heated DNA, inhibited the action of ethidium chloride on viruses. Toluidine blue and neutral red were more strongly inhibited by heated DNA than by native DNA, and both DNAs inhibited the action of proflavine to the same extent. Toluidine blue and proflavine catalysed the degradation of guanine by light in both native and heated thymus DNA. The action of neutral red differed in two respects: at a low ratio of dye to nucleotide molecules, the cytosine of native DNA was also degraded; at a higher ratio, only the guanine of heated DNA was destroyed. There was no effect of ethidium chloride on any of the four bases of DNA.