Structural Elements of Balbiani Ring BRa Of Chironomus Thummi

Abstract

To study the molecular-cytological organisation of the tissue specific puff Balbiani ring BRa of C. thummi a number of clones from a microlibrary of a part of the fourth chromosome were hybridized in situ to polytene chromosomes. Clones, containing insertions homologous to the DNA of BRa, were identified. The nucleotide sequence of some of them (F6.2; C1.2; C6–10) were determined. The length of the F6.2 is 2333 bp, the content of AT pairs is 63%; the fragment C1.2 is 2040 bp long, with 67% AT pairs. Comparative computer analysis revealed some similarity between these fragments. They include the next structural elements: a) coding regions (ORF) with promoter zone, b) tandem direct imperfect repeats of two types, c) pseudogene, d) mobile element (MEC). Schematically these elements are represented in Figure 1. The length of the coding region α is 723 bp in the F6.2 and 738 bp in the C1.2. Both regions are preceded by zones containing signal sequences for RNA-polymerase II and occupying the same topographic positions. At the nucleotide level these zones are to some degree homologous to the promoter of the globin gene of C. thummi (Trewitt et al., 1988). The α regions are flanked by direct repeats R1, 21 bp each. Units are repeated 8 times in the F6.2 and 13 times in the C1.2 fragments. In the case of the F6.2 the open reading frame (ORF) is terminated into the first of these R1 repeats, but it can be extended from the β region.