Z-DNA in transcriptionally active chromosomes.

Abstract

Due to the striking correlation between the distribution of transcriptionally active subdivisions of the polytene chromosomes and Z-DNA, we have addressed the question of whether the Z-DNA configuration exists in native, transcriptionally active chromosomes of Drosophila hydei prepared without interference by procedures known to induce the B to Z conformation. Our experiments indicate that Z-DNA forms are present in a specific set of sites on the native chromosomes. They occur on interbands and other subdivisions of dispersed DNA, but there is no correlation between the amount of Z-DNA detected and DNA compaction. The results suggest, moreover, that Z-DNA forms are restricted to specific genes, because various subdivisions induced to transcription in puffs show different patterns of Z-DNA. We show, in addition, that removal of chromosomal proteins by proteinase K has a strong influence on the level of anti-Z-DNA reactivity.In recent years the concept that DNA is not a static molecule but is a dynamic structure with considerable conformational flexibility represents a significant development in the biochemistry of DNA. In fact, soon after the formulation of the B-DNA model there was some discussion about the possible existence of alternative conformational variants. This discussion, never reflected in systematic studies, was stimulated by the unexpected discovery of a left-handed structure in a crystal of an alternative hexanucleotide (dC-dG)3 (1, 2). The discovery initiated an intensive search for the possible biological significance of this conformation, named Z-DNA. The observations (i) that the transition from B to Z may occur in conditions related to hydration of the molecule (3)(4)(5) or to the type and number of the associated cations, (ii) that it is not restricted to poly(dC-dG)3 (6-11), and (iii) the possibility that in vivo Z-DNA can be stabilized by negative supercoiling (7,(12)(13)(14)(15) and specific binding proteins (16)

The Z-DNA motif d(TG)30 promotes reception of information during gene conversion events while stimulating homologous recombination in human cells in culture.

“…It they readily adopt the left-handed Z-DNA conformation under the appropriate conditions in vitro (14,30,33), and there is evidence that left-handed DNA is found in vivo in transcriptionally active chromosomes (22). The existence of Z-DNA-binding proteins in Drosophila melanogaster and wheat germ (21) and in mammalian cells (8) supports the notion that the left-handed DNA sequences do serve some biological function.…”

A Group of Chromosomal Proteins Is Specifically Released by Spermine and Loses DNA-Binding Activity upon Phosphorylation

“…The fact that spermidine is less effective at inducing Z-DNA and putrescine is ineffective correlates with our results. A potential site of action could be one of the repetitive sequences [d(TG)n] that is found in intergenic regions and that readily forms Z-DNA in vitro under appropriate conditions (Singleton et al, 1982), and possibly also in vivo (Lancillotti et al, 1987). Such a transition of right-handed B-DNA to lefthanded Z-DNA will create positive supercoiling along native or chromatinized DNA.…”

The Biology of Left-handed Z-DNA

“…Antibodies raised in rabbits and sheep were used in staining experiments with both fixed (28) and unfixed polytene chromosomes of Drosophila (29). These produced an unusual pattern with staining in the interband regions but not in the bands.…”