A molecular and cytogenetic analysis of λ 20p7 fragment DNA from the proximal β-heterochromatin of Drosophila melanogaster

Abstract

A DNA fragment from the Drosophila melanogaster genome, cloned in /g120p7, was derived independently from clones /g120 and λL [Baiborodin et al., Genetika 29 (1993) 403–416; Sharakhov et al., Genetika 29 (1993) 392–402]. In situ hybridization of λ.20p7 DNA to the chromosomes of D. melanogaster demonstrated preferential hybridization of the fragment to the chromocenter of polytene chromosomes and to pericentric heterochromatin of chromosomes II, IV and X at the metaphase plate. Copy number per haploid genome for /g120p7 was estimated as approximately 200. Based on Southern blotting, the major portion of this moderate repeat was localized in the region of a 5.5-kb HindIII digest. In situ hybridization to polytene chromosomes from strain fs(2)β trophocytes revealed that repeats homologous to A,20p7 are located in the proximal heterochromatin which undergoes structural reorganization during tissue differentiation. The nucleotide sequence of two segments of the clone /g120p7, Dm0.9 and Dm270, was determined. Sequence analysis of the 300-bp Dm0.9 clone revealed that it contains 21-bp and 30-bp d(GT/CA) sequences, a 12-bp AT box, recognition sites for nuclear factors NFI and Spl, and a set of inverted repeats. Clone Dm270 contains an open reading frame (ORF). The deduced amino acid (aa) sequence shares homology with the gag-like gene from type-I (Rl) ribosomal DNA insertion and may code for a polypeptide of 10 kDa. The Dm270 sequence was found to contain two direct repeats showing homology to the human CENP-B box.