A. Rich

(dC―dA)n•(dG―dT)n sequences have evolutionarily conserved chromosomal locations in Drosophila with implications for roles in chromosome structure and function

In situ hybridization of (dC‐dA)n.(dG‐dT)n to the polytene chromosomes of Drosophila melanogaster reveals a clearly non‐random distribution of chromosomal sites for this sequence. Sites are distributed over most euchromatic regions but the density of sites along the X chromosome is significantly higher than the density over the autosomes. All autosomes show approximately equal levels of hybridization except chromosome 4 which has no detectable stretches of (dC‐dA)n.(dG‐dT)n. Another striking feature is the lack of hybridization of the beta‐heterochromatin of the chromocenter. The specific sites are conserved between different strains of D. melanogaster. The same overall chromosomal pattern of hybridization is seen for the other Drosophila species studied, including D. simulans, a sibling species with a much lower content of middle repetitive DNA, and D. virilis, a distantly related species. The evolutionary conservation of the distribution of (dC‐dA)n.(dG‐dT)n suggests that these sequences are of functional importance. The distribution patterns seen for D. pseudoobscura and D. miranda raise interesting speculations about function. In these species a chromosome equivalent to an autosomal arm of D. melanogaster has been translocated onto the X chromosome and acquired dosage compensation. In each species the new arm of the X also has a higher density of (dC‐dA)n.(dG‐dT)n similar to that seen on other X chromosomes. In addition to correlations with dosage compensation, the depletion of (dC‐dA)n.(dG‐dT)n in beta‐heterochromatin and chromosome 4 may also be related to the fact that these regions do not normally undergo meiotic recombination.

Anti-Z-DNA antibody binding can stabilize Z-DNA in relaxed and linear plasmids under physiological conditions.

It is shown that anti‐Z‐DNA antibody binding can stabilize sequences of d(CG/GC)n and d(CA/GT)n in the Z‐DNA conformation in a plasmid in the complete absence of supercoiling. This effect is quantitated by using antibody preparations of different affinities and varying concentrations. The d(CG/GC)n sequence can be stabilized under physiological conditions. This is the first demonstration that a region of Z‐DNA can be stabilized by protein binding in a completely relaxed plasmid under physiological conditions. The antibody‐Z‐DNA complex in the relaxed plasmid is shown to be an equilibrium state and not a long‐lived kinetic intermediate since specific binding of the antibody to linearized plasmids containing Z‐forming sequences is observed.

Z-DNA and the Polytene Chromosome

Indirect immunofluorescence shows that antibodies specific for the Z conformation of DNA bind to D. melanogaster polytene chromosomes in a specific and reproducible pattern. In our acetic-acid-fixed preparations, the binding is predominantly to interband regions, to puffs, and to some specific bands. As might be expected, the pattern of binding can be affected by cytological procedures. Bands and interbands show significant differences in their susceptibility to these procedures. Although further study is needed, our studies give no evidence that acetic acid treatment of intact cells is capable of causing conversion of B-form and Z-form DNAs. Instead, the experiments suggest that the Z conformation is preferentially available in interbands and a few other chromosome regions.