Effects of adenine tracts on the B-Z transition. Fine tuning of DNA conformational transition processes.

The effects exerted by short runs of adenines (A-tracts), alternating (AT)n segments, and single-stranded DNA upon the right- to left-handed DNA transition, as well as upon the energetic and structural parameters of the B/Z junctions, were investigated by using synthetic segments in which these motifs are coupled to a potentially Z-forming core. UV, CD, and 31P NMR studies of the salt-induced B to Z transition occurring in the various segments indicate that the transition is composed of two phases: a slow rate-determining induction of an initial structural deformation followed by a cooperative propagation of this "nucleus" in the form of a left-handed Z-DNA. The first phase is found to be crucially affected by the nature of the sequences coupled to the potentially Z-forming core. Thus, a higher rigidity of the flanking segments, such as that characterizing adenine tracts, is associated with higher energy values required for the induction of the initial conformational deformation, as well as with more defined structural parameters of the ultimate B/Z junctions. The second phase is affected mainly by the composition and sequence of the Z-forming segment. The observations that DNA conformational changes can be finely tuned and modulated by parameters pertaining to both the segment which undergoes the transition and the flanking sequences support the notion that DNA secondary motifs, such as the Z form and A-tracts, might be involved in the regulation of cellular processes.