The binding of actinomycin to crab dAT; the nature of the DNA binding site

Actinomycin Binding to DNA: Inability of a DNA Containing Guanine To Bind Actinomycin D

Polydeoxy (adenylyl-thymidylyl-cytidylyl)* polydeoxy(guanylyl-adenylyl-thymidylyl), a double-stranded DNA polmer of high molecular weight containing 33 percent guanine plus cytosine, binds little or no actinomycin D, as measured by five different techniques. In contrast, the sequence isomer of this DNA, polydeoxy (thymidylyl-adenylyl-cytidylyl)-polydeoxy(guanylyl-thymidylyl-adenylyl), does bind the antibiotic. Thus, the presence of guanine in a DNA is not a sufficient requisite for the binding of actinomycin D.

Further experiments on the effect of actinomycin D on phytochrome-mediated anthocyanin synthesis

Phytochrome-mediated anthocyanin synthesis of the mustard seedling can be blocked by moderate concentrations of Actinomycin D (10 μg/ml in the solution around the seedling) if the substance is applied before the onset of far-red light (Fig. 1). If, however, the seedlings are irradiated with far-red for 6 hours, transferred to the dark for one hour, incubated with Act. D for 3 hours (in the dark) and then re-irradiated with continuous far-red, anthocyanin synthesis can only partially be inhibited (Fig. 2). - There are good arguments that the physiological action of the small stationary concentration of P730 which is left in the plant after we turn off the far-red light will virtually stop at the moment when the light is turned off (WAGNER and MOHR, 1966a). Furthermore the P730-dependent mRNAs which are involved in anthocyanin synthesis seem to be shortlived (LANGE and MOHR, 1965; MOHR and SENF, 1966; DURST and MOHR, 1966). Considering these arguments and the results of the present paper we cannot but conclude that genes which have once been activated by P730 will not return to the original state-at least with respect to Act. D sensitivity-even when the activating (or de-repressing) agent (P730 in our case) has disappeared.