THE UNUSUAL X-FORM DNA IN OLIGODEOXYNUCLEOTIDES - DEPENDENCE OF STABILITY ON THE BASE SEQUENCE AND LENGTH

X-form is an unusual double helix of DNA adopted by poly(dA-dT) or (dT -dA)(4) at high concentrations of CsF. On the other hand, poly(dA).pol y(dT), (dA-dT)(4) and most other DNAs do not adopt this conformer. Her e we demonstrate that the X-form is strongly destabilized by GC pairs or even minute perturbations of the alternating pyrimidine-purine sequ ence. For example, the 30-mer d(TATAAT)(5), containing five tandem rep eats of the Pribnow box, fails to isomerize into the X-form. After (dT -dA)(4), the 16-mer (dT-dA)(8) is shown to be the second most predispo sed oligodeoxynucleotide in the (dT-dA)(n) series to isomerize into th e X-form while the duplex lengths corresponding to n=3,5,6,7,9,12 and 20 make the X-form unstable even in the strictly alternating (dT-dA)(n ) sequence. Consequently, the (dT-dA)(n) duplex length is also a cruci al factor of the X-form stability on the oligodeoxynucleotide level. W e discuss a possibility that the X-form is a solution counterpart of t he D-form adopted in dehydrated poly(dA-dT) fibers because properties of these two conformers are remarkably similar in many respects.