Elsamicin A can convert the Z-form of poly[d(G-C)] and poly[(G-m5C)] back to B-form DNA.

The interaction of poly(G-C)] and polyd(G-m5C)] with the antitumor antibiotic elsamicin A, which binds to alternating guanine + cytosine tracts in DNA, has been studied under the B and Z conformations. Both the rate and the extent of the B-to-Z transition are diminished by the antibiotic, as inferred by spectroscopic methods under ionic conditions that otherwise favor the left-handed conformation of the polynucleotides. Moreover, elsamicin converts the Z-form DNA back to the B-form. The circular dichroism data indicate that elsamicin binds to polyd(G-C)] and polyd(G-m5C)] to form a right-handed bound elsamicin region(s). The transition can be followed by changes of the molar ellipticity at 250 nm, thus providing a convenient wavelength to monitor the Z-to-B conformational change of the polymers as elsamicin is added. The elsamicin A effect might be explained by a model in which the antibiotic binds preferently to a B-form DNA, playing a role as an allosteric effector on the equilibrium between the B and Z conformations, thus favoring the right-handed one.