Saframycin Mx2

DNA sequence selectivities in the covalent binding of the antitumor antibiotic saframycins Mx1, Mx3, A, and S have been determined by complementary strand MPE.Fe(II) footprinting and exonuclease III stop assays on two different 545 and 135 base pair long HindIII/RsaI restriction fragments of pBR322 DNA. Saframycins Mx1, Mx3, A, and S recognize primarily 5'-GGG sequences. All four antibiotics also recognize 5'-GGPy sequences, however a cytosine is preferred over a thymine at the 3'-end of this recognition site in all cases. Saframycins Mx1, Mx3 and S, which possess the OH leaving group, also recognize the 5'-CCG sequence, in contrast to saframycin A, which contains the CN leaving group. In contrast, the OH-containing saframycins also recognize the 5'-CTA sequence. Saframycins Mx2, B and C, which lack the critical CN or OH leaving group, do not show any footprints on the restriction fragments examined in this study. The measured binding site size for all four antibiotics is three base pairs. The exonuclease III stop assay independently confirmed the formation of a covalent bond and the strong preference of the antibiotics for 5'-GGG and 5'-GCC sequences. The latter enzyme assay also suggests that the 5'-terminal or central G of the triad binding site is the base to which reversible covalent attachment of the antibiotic takes place.