UNUSUAL DNA STRUCTURES, CHROMATIN AND TRANSCRIPTION

Extensive studies of DNA secondary structure during the past decade ha ve shown that DNA is a dynamic molecule, whose structure depends on th e underlying nucleotide sequence and is influenced by the environment and the overall DNA topology. Three major non-B-DNA structures have be en described (Z-DNA, tripler DNA and cruciform DNA) which are stabiliz ed by unconstrained negative supercoiling and can be formed under phys iological conditions. In this essay we summarize the DNA primary struc ture features that are pertinent to the formation of these conformers and present data concerning the occurrence of these-sequences in the e ukaryotic genome. The evidence in favor of the existence of these unus ual DNA structures in vivo is discussed. The effect of alternative non -B-DNA structures on the way DNA is organized in chromatin is consider ed, and this is followed by evaluation of the data relating these stru ctures to eukaryotic transcription. Some possible mechanisms by which the effect of non-B structures on transcription might be exerted are p roposed.