Ultrastructure of polytene chromosomes of Drosophila isolated by microdissection

Polytene Chromosomes: 70 Years of Genetic Research

Polytene chromosomes were described in 1881 and since 1934 they have served as an outstanding model for a variety of genetic experiments. Using the polytene chromosomes, numerous biological phenomena were discovered. First the polytene chromosomes served as a model of the interphase chromosomes in general. In polytene chromosomes, condensed (bands), decondensed (interbands), genetically active (puffs), and silent (pericentric and intercalary heterochromatin as well as regions subject to position effect variegation) regions were found and their features were described in detail. Analysis of the general organization of replication and transcription at the cytological level has become possible using polytene chromosomes. In studies of sequential puff formation it was found for the first time that the steroid hormone (ecdysone) exerts its action through gene activation, and that the process of gene activation upon ecdysone proceeds as a cascade. Namely on the polytene chromosomes a new phenomenon of cellular stress response (heat shock) was discovered. Subsequently chromatin boundaries (insulators) were discovered to flank the heat shock puffs. Major progress in solving the problems of dosage compensation and position effect variegation phenomena was mainly related to studies on polytene chromosomes. This review summarizes the current status of studies of polytene chromosomes and of various phenomena described using this successful model.

Cloning and molecular genetic analysis of Drosophila melanogaster interband DNA

Interband DNA of Drosophila melanogaster polytene chromosomes was studied using a novel approach based on the electron microscopic (EM) analysis of chromosome regions carrying DNA fragments of known molecular genetic composition, inserted by P element-mediated transformation. Insertion of such fragments predominantly into interbands makes it possible to clone interband DNA by constructing genomic libraries from transformed strains and probing them with the insert DNA. The transformed strain P[H-sp70:Adh](61C) has insertion in the 61C7-8 interband on the left arm of chromosome 3. This DNA consists of part of the hsp70 gene promoter fused to the coding region of the Adh gene, and is flanked on either side by P element sequences. We constructed a genomic library from DNA of this strain and isolated a clone containing the insert and the interband DNA. Subsequently the genomic library of wild-type strain was probed with a subclone composed of interband DNA only. We have thus isolated a clone containing the entire native interband. 1289 bp of interband DNA was sequenced and found to be AT-rich (53.4%) with numerous regions of overlapping direct and inverted repeats, regulatory sites, and two overlapping open reading frames (ORFs).

Bands, interbands and puffs in native Drosophila polytene chromosomes are recognized by a monoclonal antibody to an epitope in the carboxy-terminal tail of histone H1

A monoclonal antibody was raised against Drosophila melanogaster histone H1. Immunoscreening of proteolytic cleavage fragments of H1 and of a set of all possible overlapping synthetic octapeptides corresponding to the amino acid sequence of H1, revealed that the antibody recognizes an epitope within the sequence 207VTAAKPKA214 near the centre of the carboxy-terminal tail. This antibody gives positive immunofluorescence over the entire length of native D. melanogaster polytene chromosomes isolated from salivary glands by microdissection at physiological pH and ionic strength. Bands, interbands and puffs are all seen to contain H1. The immunofluorescence over puffs, albeit lower than that over bands and interbands, indicates that chromatin decondensation can occur without complete loss of H1 in these structures. The reaction of the antibody with bands suggests that the segment of the C-terminal tail containing the epitope may be exposed in the condensed 30 nm chromatin filament.

The ultrastructural morphology of native salivary gland chromosomes of Drosophila melanogaster: the band-interband question

Native salivary gland chromosomes of Drosophila melanogaster, isolated without exposure to acid fixatives, have been examined in regions 1A-3B, 15A-17B, 19B-20D and 71E-73A and reveal improved aspects of preservation at the ultrastructural level. Three main points emerge: fine bands are well preserved allowing detection of some not recorded in maps made on classical acid-fixed preparations. Structures with the morphology of putative nascent ribonucleoprotein (RNP) particles are apparent in puffs, diffuse bands and virtually all interbands observed. At this level the morphology of native chromosomes is consistent with the hypothesis that all decondensed regions are members of a continuum of transcriptionally active structures. This notion is relevant to data obtained from other approaches to the band-interband question. (iii) Although the chromosomes have not been exposed to 45% acetic acid, at least some of the dark bands represented by the Bridges as doublets in their classical maps contain vacuoles which include putative RNP particles.

Z-DNA immunoreactivity of Drosophila polytene chromosomes. Effects of the fixatives 45% acetic acid and 95% ethanol and of DNase I nicking

Drosophila salivary chromosomes have been isolated at neutral pH and physiological ionic strength. They display only background level binding of antibodies against Z-DNA. Following exposure to the commonly used fixative 45% acetic acid all of the polytene chromosomes, X and autosomes, show a massive increase in anti-Z-DNA antibody binding. The enhancement from background to intense fluorescence occurs whether the chromosomes are stabilised by two orders of magnitude lower concentration of formaldehyde than that used to minimise protein extraction in classical acid squash preparations, or by physiological concentrations of spermine and spermidine. Nicking of acetic acid-treated chromosomes by DNase I dramatically reduces their Z-DNA immunoreactivity. The histones and non-histones extracted by 45% acetic acid from unfixed and formaldehyde-fixed Drosophila chromatin have been analysed. Exposure of isolated salivary chromosomes to the non-protein-extracting fixative 95% ethanol also enhances Z-DNA immunoreactivity. All of these phenomena must be taken into account in the search for the Z-DNA conformation in cells by cytological techniques.

Dependence of Z-DNA antibody binding to polytene chromosomes on acid fixation and DNA torsional strain

There is considerable interest in the existence and significance of alternative conformations of DNA to the right-handed B-form described originally by Watson and Crick. The indirect immunofluorescence observations of Nordheim et al., Arndt-Jovin et al. and Lemeunier et al. that antibodies against left-handed Z-DNA bind to polytene chromosomes have thus assumed considerable importance. However, there is a paradox: some workers observe Z-DNA in interbands and others in bands. We report here that binding of Z-DNA antibodies to Drosophila polytene chromosomes prepared without acid fixation is at background level, and that following acid fixation the same antibody treatment leads to intense fluorescence. Depending on the extent of exposure to 45% acetic acid, fluorescence can occur primarily in interbands or in bands. Furthermore, antibody binding is dependent on elastic torsional strain in the DNA molecules.