THE DETECTION OF B-FORM A-FORM JUNCTION IN A DEOXYRIBONUCLEOTIDE DUPLEX/

The transition of the 14-meric deoxyoligonucleotide duplex d-(ACCCCCTT TTTTTG)-d-(CAAAAAAAGGGGGT) from the B- to the A-conformation in water/ trifluorethanol (TFE) solution was studied with the use of circular di chroism. An increase in the fraction of TFE induces a two-step B-A tra nsition. in the first step, up to 73% TFE, the A-form is generated fro m the GC-rich part; in the second step, 73-82% TFE, the AT-rich part s hifts to the A-form. By this we suggest the existence of a BIA junctio n near 73% TFE. Emergence of the BIA junction has been directly confir med with the use of distamycin A and netropsin, ligands known to selec tively bind to AT stretches of B-DNA. it can be shown that both ligand s suppress formation of the A-form in the B-philic part. The free ener gy value for the B/A junction was estimated to be 2.1. kcal/mol, which agrees well with known data for polymeric DNAs. The obtained results may have biological relevance in connection with recently published x- ray data about the occurrence of the B/A junction in the complex of DN A with reverse transcriptase of HIV.