Structural and Functional Analysis of Balbiani Ring Genes in the Salivary Glands of Chironomus tentans

Abstract

During recent years, much attention has been devoted to studies of the regulation of gene activity in eukaryotic cells. Individual eukaryotic genes have been characterized by the new recombinant DNA technology and defined sequences have been tested for functions in both in vitro (Wasylyk et al., 1980) and in vivo systems (Grosschedl and Birnstiel, 1980). Furthermore, the rapidly expanding knowledge of chromosome structure suggests that the chromosomal fiber and its organization into higher order structures is probably important for the regulation of gene activity (Finch and Klug, 1976; Sedat and Manuelidis, 1978, Suau et al., 1979). Certain results even indicate that a change in chromosome structure could, in fact, be the primary event upon gene activation in eukaryotes (e.g., Foe, 1978). The structure of inactive chromatin is rather well understood (Kornberg, 1977; Chambon, 1978; Felsenfeld, 1978; Klug et al., 1980), while less information is available on the structure of the fiber in the active state (Mathis et al.,1980). It has been demonstrated in micrococcal digestion experiments that the active fiber maintains the 200 base pair (bp) subunit structure characteristic for inactive chromatin (Bellard et al., 1978). Other biochemical data show, however, that the chromosome fiber is altered upon transcription; an increased sensitivity to DNase I has, for example, been noted (Weintraub and Groudine, 1976). The main obstacle in the biochemical investigations of transcriptionally active chromatin is that this fraction of chromatin is a minor one and cannot easily be isolated from total chromatin (for discussion, see Mathis et al., 1980). Electron microscopy offers here a distinct advantage since active genes can be directly studied by this method. It has even been feasible to investigate two specific active genes, the silk fibroin gene in the spinning glands of Bombyx mori (McKnight et al., 1976) and the Balbiani ring 75 S RNA genes in the salivary glands of Chironomus tentans (Lamb and Daneholt, 1979).