THE EFFECT OF ZINC ON THE SECONDARY STRUCTURE OF D(GA.TC)N DNA-SEQUENCES OF DIFFERENT LENGTH - A MODEL FOR THE FORMATION ASTERISK-H-DNA

Alternating d(GA.TC)n DNA sequences are known to undergo transition to H-DNA In the presence of zinc. Here, the effect of zinc on the secon dary DNA structure of d(GA - TC)n sequences of different length (n = 5 , 8, 10 and 19) was determined. Short d(GA.TC)n sequences form H-DNA with a higher difficulty than longer ones. At bacterial negative super helical density (- sigma = 0.05), zinc still induces transition to the H-DNA conformation at a d(GA.TC)10 sequence but shorter sequences d o not form H-DNA. Transition to *H-DNA at a d(GA-TC)8 sequence is ob served under conditions which destabilize the DNA double helix such as high negative supercoiling or low ionic strength. Our results indicat e that a first step in the transition to H-DNA is the formation of a denaturation bubble at the centre of the repeated DNA sequence, sugges ting that the primary role of zinc is to induce a local denaturation o f the DNA double helix. Subsequently, zinc might also participate in t he stabilization of the altered DNA conformation through its direct in teraction with the bases. Based on these results a model for the forma tion of H-DNA is proposed.