EFFECT OF FLANKING SEQUENCES ON THE RIGHT-HANDED TO LEFT-HANDED TRANSITION OF A (DA-DT)(N) TRACT IN SUPERCOILED DNA

Alternating (dA-dT)(n) sequences in supercoiled DNA may undergo a tran sition to a left-handed conformation in the presence of Ni2+ ions and high NaCl concentration (Nejedly, K., Klysik, J. and Paleicek, E., FEE S Lett. 243, 313-317 (1989)). In this work we have found that ionic co nditions necessary for the B-to-Z transition are strongly dependent on the sequences flanking the (dA-dT)(n) tract. In particular, the prese nce of 5'- homopyrimidine (C3) and 3'-homopurine (G4) blocks adjacent to the tract were found to facilitate the transition to the left-hande d form. Within a constant sequence context it was found that the ionic strength required to promote the transition was inversely proportiona l to the length of the (dA-dT)(n) sequence.