Polytene chromosome

Figure 36-4. Illustration of the tight correlation between the presence of RNA polymerase II and RNA synthesis. A number of genes are activated when Chirono-mus tentans larvae are subjected to heat shock (39 °C for 30 minutes). A Distribution of RNA polymerase II (also called type B) in isolated chromosome IV from the salivary gland (at arrows). The enzyme was detected by immunofluorescence using an antibody directed against the polymerase. The 5C and BR3 are specific bands of chromosome IV, and the arrows indicate puffs. B Autoradiogram of a chromosome IV that was incubated in H-uridine to label the RNA. Note the correspondence of the immunofluorescence and presence of the radioactive RNA (black dots). Bar = 7 pm. (Reproduced, with permission, from Sass H RNA polymerase B in polytene chromosomes. Cell 1982 28 274. Copyright 1982 by the Massachusetts Institute of Technology.)...

Thoma F, Roller T (1977) Influence of histone HI on chromatin structure. Cell 12 101-107 Thoma F, Roller T, Klug A (1979) Involvement of histone HI in the organization of the nucleosome and of the salt-dependent superstructures of chromatin. J Cell Biol 83 403 27 Turner BM, Birley AJ, Lavender J (1992) Histone H4 isoforms acetylated at specific lysine residues define individual chromosomes and chromatin domains in Drosophila polytene nuclei. Cell 69 375-384... 

CHDl (Drosophila) SSRPl 113 Colocalisation of CHDl and SSRPl on polytene chromosomes... 

The first indication that modification of specific tail residues were linked to chromatin functional states, came from immunostaining of Drosophila polytene chromosomes with antibodies specific for H4 acetylated at defined lysines [13]. As shown in Fig. 2A, H4 acetylated at lysine 16 (H4acK16) was found almost exclusively on the transcriptional hyperactive male X chromosome (Fig. 2). (Genes on the Drosophila male X are transcribed twice as fast as their female counterparts so as to equalize levels of X-linked gene products between XY males and XX females.) In addition, H4 lysine 12 was found to remain acetylated in centric heterochromatin, while lysines 5, 8, and 16 were all under-acetylated [13]. These observations led to the suggestion that the histone N-terminal tails constitute nucleosome surface markers that can be recognized by non-histone proteins in a modification-dependent manner to alter the functional state of chromatin [13]. 

Turner, B.M., Birley, A.J., and Lavender, J.S. (1992) Histone H4 isoforms acetylated at specific lysine residues define individual chromosomes and chromatin domains in Drosophila polytene nuclei. Cell 69, 375-384. 

Another use of polytene chromosomes is microdissection of DNA for cloning (Fig. 26-14). A piece of DNA containing 100-400 kb can be cut out of any desired spot, cleaved with restriction enzymes, and cloned.282 Since it has been estimated that Drosophila may contain only -9000-17,000 genes there may be 2-3 genes per band in these chromosomes.283 The technique has been extended to human and other mammalian chromosomes.284... 

Like the puffs of polytene chromosomes (Chapter 28), which may have a similar structure, lampbrush chromosomes appear to be actively engaged in transcription. Approximately 3% of the DNA may be functional in producing mRNA that is accumulated within the oocyte and is used as a template for protein synthesis during early embryonic development.90... 

Stress proteins. Ritossa reported in 1962 that when Drosophila were suddenly warmed from 25°C to 36-37°C, a series of new puffs could be observed within the polytene chromosomes of the salivary glands.442 These same puffs, which were also seen after other stresses, appeared within a few minutes and were associated with formation of new mRNAs. This heat-shock phenomenon was subsequently found to be universal. It is observed in all species of animals as well as in plants and bacteria.443 48... 

Ecdysone stimulates the synthesis of RNA in tissues. Visual demonstration of the effect is provided by its action on polytene chromosomes of fly larvae (Fig. 26-14).361 Fifteen minutes after the application of ecdysone, a puff is induced on one band of the chromosome a second puff forms at a later time while a preexisting puff diminishes. Thus, like steroid hormones in mammals, ecdysone appears to have a direct controlling effect on transcription. The cuticle-shedding process (ecdysis) is initiated by the brain peptide eclosian. However, the brain may be responding to the ecdysis-triggeiing hormone, a peptide that is secreted by a series of epitracheal glands located in various segments of the body.362... 

Transcription in polytene chromosomes usually is associated with local swellings of the bands, called puffs (see... 

Ashbumer M., Chihara C., Meltzer P. and Richards G. (1973) Temporal control of puffing activity in polytene chromosomes. Cold Spring Harbor Symp. Quant. Biol. 38, 655-662. 

Young7 screened 80 randomly isolated plasmids from a genomic library of D. melanogaster DNA for repetitive elements. Of these 23 were found to be repetitive by hybridization to Southern blots of D. melanogaster DNA, and 17 of these were interspersed repetitive elements as shown by in situ hybridization with polytene chromosomes. 

Nordheim A, Pardue ML, Lafer EM, MOller A, Stollar D, Rich A (1981) Antibodies to left-handed Z-DNA bind to interband regions of Drosophila polytene chromosomes. Nature (Lond) 294 417-422... 

This gene codes for an 0.8 kb mRNA and conceptual translation of the cochng region revealed a peptide that shows 69% identity to Manduca EH. In situ hybridization to the polytene chromosomes from larval salivary glands showed that the Drosophila EH gene is located on the right arm of chromosome 3 at location 90B. This region does not correspond to any known mutations, but stocks are available with deletions in this area to facilitate a screen for mutants at the EH locus. 

While Drosophila is certainly the insect which has been most intensively studied, it is worth mentioning that genetic studies may become important in other genera as we struggle with problems of insecticide resistance, for example. Fortunately, other insects do have some of the features which make Drosophila favorable for study, such as polytene chromosomes, and it may be that recombinant DNA research combined with such features will encourage much more work in other species. 

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