Earlier studies by others had indicated that actinomycin D (ACTD) binds well to d(AACCATAG) and the end sequence TAG-3' is essential for its strong binding. In an effort to verify these assertions and to uncover other possible strong ACTD binding sequences as well as to elucidate the nature of their binding, systematic studies have been carried out with oligomers of d(AACCAXYG) sequence motifs, where X and Y can be any DNA base. The results indicate that in addition to TAG-3', oligomers ending with XAG-3' and XCG-3' all provide binding constants > or =1 x 10(7) M(-1) and even sequences ending with XTG-3' and XGG-3' exhibit binding affinities in the range 1-8 x 10(6) M(-1). The nature of the strong ACTD affinity of the sequences d(A1A2C3C4A5X6Y7G8) was delineated via comparative binding studies of d(AACCAAAG), d(AGCCAAAG) and their base substituted derivatives. Two binding modes are proposed to coexist, with the major component consisting of the 3'-terminus G base folding back to base pair with C4 and the ACTD inserting at A2C3C4 by looping out the C3 while both faces of the chromophore are stacked by A and G bases, respectively. The minor mode is for the G to base pair with C3 and to have the same A/chromophore/G stacking but without a looped out base. These assertions are supported by induced circular dichroic and fluorescence spectral measurements.