The early events of Drosophila metamorphosis were first elucidated after seeing the effects of ecdysone on polytene chromosomes. During Drosophila molting and metamorphosis, certain regions of these chromosomes “puff out” in the cells of certain organs at particular times (Figure 1; Clever 1966; Ashburner 1972; Ashburner and Berondes 1978). These chromosome puffs are areas where DNA is being actively transcribed. When 20E is added to larval salivary glands, certain puffs are produced and others regress. Fluorescent antibodies against 20E demonstrated that the hormone localizes at the puff sites, further confirming the involvement of 20E in transcription of target genes at the puff sites (Gronemeyer and Pongs 1980). At these sites, the ecdysone-bound receptor complex recruits a histone methyltransferase that methylates lysine-4 of histone H3, thereby loosening the nucleosomes in that area (Sedkov et al. 2003). Remarkably, after the initial puffs had formed, other puffs were seen to regress. And hours later, more puffs formed. Ashburner (1974, 1990) hypothesized that the “early puff” genes make a protein product that is essential for the activation of the “late puff” genes and that, moreover, the early regulatory protein itself turns off the transcription of the early puff genes.1 These insights have been confirmed by molecular analyses. Figure 21.11 shows a simplified schematic for the framework of metamorphosis in Drosophila.
Ashburner, M. 1972. Patterns of puffing activity in the salivary glands ofDrosophila. VI. Induction by ecdysone in salivary glands of D. melanogastercultured in vitro. Chromosoma 38: 255–281.
Ashburner, M. 1974. Sequential gene activation by ecdysone in polytene chromosomes of Drosophila melanogaster. II. Effects of inhibitors of protein synthesis. Dev. Biol. 39: 141–157.
Ashburner, M. 1990. Puffs, genes, and hormones revisited. Cell 61: 1–3.
Ashburner, M. and H. D. Berondes. 1978. Puffing of polytene chromosomes. In The Genetics and Biology of Drosophila, Vol. 2B. Academic Press, New York, pp. 316–395.
Clever, U. 1966. Induction and repression of a puff in Chironomus tentans. Dev. Biol. 14: 421–438.
Gronemeyer, H. and O. Pongs. 1980. Localization of ecdysterone on polytene chromosomes of Drosophila melanogaster. Proc. Natl. Acad. Sci. USA 77: 2108–2112.
Sedkov, Y. and 9 others. 2003. Methylation of lysine-4 of histone H3 in ecdysone-dependent development of Drosophila. Nature 426: 78–83.
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1. The observation that 20E controlled the transcriptional units of chromosomes was an extremely important and exciting discovery. One could see transcription occurring, using only a light microscope. This was our first real glimpse of gene regulation in eukaryotic organisms. At the time when this discovery was made, the only examples of transcriptional gene regulation were in bacteria.