Active genes and puffs

Abstract

Present knowledge about the structure and function of polytene chranosomes has been extensively reviewed by Beermann (1956, 1972), by Berendes (1972) and by Ashburner (1972). The reader may also consult a more recent review about the informational content of polytene chromosome bands and puffs written by Zhimulev et al. (1981). I will only discuss the relationship of puffs to active genes, specifically the ecdysteroid regulated patterns of puffing and gene expression, which can be studied in salivary glands of third instar larvae of Drosophila melanogaster. When ecdysteroid hormone canes into contact with salivary glands of third instar larvae of Drosophila melanogaster, a number of puffs is sequentially activated, others are inactivated (Ashburner, 1972). The first consequences of the hormone’s action include the repression of puffing at those loci concerned with the synthesis of the salivary gland secretory proteins and the induction of puffing at several specific “early” sites, most nobably at loci 285, 74ET and 75B. There are many other puffs, which respond to the hormone’s presence after a lag period of at least 3 to 4 hr, i.e. at the time, when early puffs regress. The ecdysteroid response of early and late puffs differs in several additional aspects.