Binding of daunomycin to diaminopurine- and/or inosine-substituted DNA

The binding of the anticancer drug daunomycin to double-helical DNA has been investigated by DNase I footprinting and fluorescence titration, using a series of polymerase chain reaction (PCR) synthesized DNA fragments that contained systematic base substitutions to alter the disposition of functional groups within the minor groove. The 160 bp tyrT DNA fragment constituted the starting material. Fragments in which (i) inosine was substituted for guanosine, (ii) diaminopurine was substituted for adenine, and (iii) both inosine and diaminopurine were substituted for guanosine and adenine, respectively, were studied. These fragments permit the role of the 2-amino group in the minor groove to be systematically explored. The results of DNase I footprinting experiments confirmed that daunomycin binds preferentially to 5'(A/T)GC and 5'(A/T)CG triplets in the normal fragment. Substitution of inosine for guanosine, with the concomitant loss of the N-2 in the minor groove, weakened binding affinity but did not dramatically alter the sequence preference associated with daunomycin binding. Complete reversal of the location of the N-2 group by the double substitution, however, completely altered the sequence preference of daunomycin and shifted its binding from the canonical triplets to ones with a 5'IDD motif. These results have critically tested and confirmed the proposed key roles of the daunosamine moiety and the 9-OH group of daunomycin in dictating binding to preferred sites. In a parallel study, both macroscopic and microscopic binding to the normal tyrT fragment were investigated, experiments made possible by using PCR to prepare large quantities of the long, defined DNA sequence. The results of these experiments underscored the complexity of the interaction of the drug with the DNA lattice and revealed unequivocal heterogeneity in its affinity for different binding sites. A class of high-affinity sites, most probably corresponding to the 5'(A/T)GC and 5'(A/T)CG triplets, was identified and characterized in macroscopic binding isotherms.