The organization of the BR2 gene locus. (A-B) A probe representing the repetitive coding sequence of the BR2.2 gene (red) was hybridized simultaneously with either the intergenic probe P1 (A) or the downstream probe P2 (B), both in green. (C) A thin section through the BR2 gene locus visualized in a transmission electron microscope. (D) A model for the organization of the chromatin bundles in the BR2 puff. The polytene chromosome is split into many gradually thinner chromatin bundles. From thin bundles, individual chromatids are unfolded extensively along the transcribed BR2.1 and BR2.2 genes. The intergenic region (detected by in situ hybridization with the P1 probe, depicted as oval green signals) is refolded into more compact chromatin, but is still located in the periphery of the gene locus. From the intergenic region, the transcribed part of the BR2.2 gene is unfolded. The red line symbolizes the in situ hybridization signal for the coding region of the BR2.2 gene, visualized with the repetitive probe. At the 3′ end of the BR2.2 gene, the downstream chromatin is compacted and gradually interact with regions from other chromatids to form thicker and larger bundles. The downstream hybridization probe signal (green triangle) labels such chromatin bundles that are more centrally located in the puff. The bars represent 5 μm in A, B and C.