Template specificities of aclacinomycin B on the inhibition of DNA-dependent RNA synthesis in vitro.

The effect of Aclacinomycin B (ACM-B), an anthracycline antitumor antibiotic, on the DNA-dependent RNA synthesis using single- and double-stranded DNAs of known base content and sequence is studied. The data show that ACM-B effectively inhibits the double-stranded DNA-directed RNA synthesis with a preference of poly[d(A-T)] greater than poly[d(G-C)] greater than poly[d(I-C)]. In contrast, it has no inhibitory effect on the template function of single-stranded DNA (e.g. poly dA, poly dT, and poly dC). These results suggest that the mechanism of ACM-B inhibition, like other anthracycline antibiotics, is by intercalation. In addition to the base specificity, there are also dramatic differences in inhibition depending on the base sequence in the DNA template. Thus, ACM-B preferentially inhibits the alternating double-stranded copolymers over the double-stranded homopolymers; e.g. poly[d(A-T)] is inhibited to a greater extent than poly dA.poly dT and poly [d(G-C)] is inhibited more than poly dG.poly dC. Since the inhibition by ACM-B can be totally abolished when assayed in excess amount of DNA, this result suggests that ACM-B inhibition of RNA synthesis is solely on the DNA template (which is in support of the intercalation model), and has ruled out the possibility that ACM-B may also exert an inhibitory effect on the activity of RNA polymerase per se.