Polytene chromosomes: The status of the band-interband question

Imaging and manipulating chromosomes with the atomic force microscope

Polytene chromosomes from the salivary gland cells of Drosophila melanogaster were examined by atomic force microscopy. The atomic force microscope (AFM) was capable of resolving chromosomal features down to the limits of the tip sharpness, about 500 A for pyramidal-shaped tips. Resolution was increased to 300 A by using electron beam deposited (EBD) tips with high aspect ratios. This significantly exceeds the resolution obtainable with conventional optical microscopes, but at the cost of compromising the structural integrity of the sample. A reasonable compromise was achieved by using oxide-sharpened tips. In this case high resolution was obtained without sample degradation, but when desired these tips were also capable of sample disintegration with increased scanning force and rate. Thus, oxide-sharpened tips were used to precisely dissect defined chromosomal regions to illustrate their potential use in genetic mapping efforts. This study illustrates the utility of the AFM in the characterization and manipulation of chromosomes and chromosomal DNA.

The reaction with polytene chromosomes of antibodies raised against Drosophila E75A protein

The steroid insect molting hormone ecdysone rapidly induces a small number of polytene chromosome puffs in Drosophila. The Ashburner model proposes that the corresponding early genes encode proteins involved in both the induction of the late genes and the repression of the early genes. The Drosophila E75 early gene has been isolated and two of its products, E75A and E75B, have been shown to be members of the steroid receptor superfamily. We have now prepared antisera directed against A- and B-specific regions of the E75 proteins. Antisera and a monoclonal antibody raised against E75A, the major larval protein product of the E75 gene, bind to discrete sites in native salivary gland chromosomes. These sites are closely correlated with early and late ecdysone responsive loci.

The faint band/interband region 28C2 to 28C4-5(-) of the Drosophila melanogaster salivary gland polytene chromosomes is rich in transcripts

Urate oxidase mRNA and five other transcripts map along 38 kb of DNA in the region 28C on the Drosophila melanogaster second chromosome. Three biotinylated restriction fragments from this 38 kb of DNA, one from each end and one from the middle, were individually hybridized in situ to slightly stretched salivary gland polytene chromosomes. The data from these in situ hybridizations in combination with the transcription map of the 38 kb of DNA indicate that: (i) there are six discrete RNA species encoded along the 38 kb of DNA and (ii) these six transcripts map to the faint band/interband region which includes the proximal edge of 28C1, the three faint bands, 28C2, 28C3 and 28C4-5(-), and the adjacent interband chromatin. Our data are consistent with the few published studies directly demonstrating that faint band/interband regions of the Drosophila melanogaster salivary gland polytene chromosomes code for a high density of transcripts.

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.