Association of chromatin with anticancer antibiotics, mithramycin and chromomycin A3.

Mithramycin and chromomycin A(3) are two anticancer antibiotics, which inhibit protein biosynthesis via transcription inhibition. They bind reversibly to DNA with (G.C) base specificity. At and above physiological pH in the absence of DNA, they form two types of complexes with Mg(2+), complex I (1:1 in terms of antibiotic: Mg(2+)) and complex II (2:1 in terms of antibiotic: Mg(2+)). These are the DNA binding ligands. In vivo, the antibiotics interact with chromatin, a protein-DNA complex. In order to understand the mode of action of these antibiotics at molecular level, we have carried out spectroscopic, gel electrophoretic and UV melting studies of complex I of these antibiotics with rat liver chromatin, nucleosome core particle and DNA stripped of all chromosomal proteins. Analysis of the results has led us to propose that the antibiotic: Mg(2+) complex binds to both nucleosomal and linker DNA in native chromatin. Histone proteins reduce the binding potential and accessibility of the complexes to the minor groove of (G.C) rich regions of chromosomal DNA. The antibiotic: Mg(2+) complex stabilizes DNA duplex and histone- DNA contacts in chromatin fiber. It also leads to the aggregation of chromatin fibers. From a comparison of the association of the antibiotic: Mg(2+) complexes with different levels of chromatin structure and their effects upon the structure, we suggest that the sugar moieties of the antibiotics play a role in the binding process. Significance of these results to understand the molecular basis of the transcription inhibition potential of the antibiotics in eukaryotes is discussed.