SEQUENCE-DEPENDENT VARIATION IN THE CONFORMATION OF DNA

The specificity of action of the enzyme DNase I on double-stranded DNA polymers of defined sequence was investigated. The results obtained with the alternating copolymers poly d(A-T) .cntdot.poly d(A-T) and poly d(G-C) .cntdot.poly d(G-C) support the suggestion of Klug et al. (1979) that regions of double-stranded DNA containing alternating purine-pyrimidine sequences may exist as structural variants of the classical B-form under physiological salt conditions. Digestion of defined oligomers containing alternating dG-dC sequences indicate that these too exist in some alternating-B structure in solution under similar conditions. The results obtained with the oligomers also provide a number of insights into the mode of action of DNase I. In the case of the B-DNA dodecamer d(C-G-C-G-A-A-T-T-C-G-C-G), for which the crystal structure has been solved (Dickerson and Drew, 1981), there is a very good correlation between the sites of rapid DNase I cutting and positions of high local helical twist.