Effect of actinomycin on a (TA)6 plasmid insert.

The effect of actinomycin on DNA structure has been studied by nuclease digestion of a DNA fragment containing the sequence AAGCT(TA)6AGCTT. The drug enhances DNase I cleavage at ApT and TpA bonds closest to the drug binding site (GC). Large changes in the relative susceptibility of bonds to cleavage by micrococcal nuclease and DNase II are also observed. The changes suggest that actinomycin alters DNA structure around its binding site, facilitating the formation of an alternating DNA structure. When a similar TA sequence was inserted further from the actinomycin binding site no changes in nuclease susceptibility were observed.