A MOLECULAR AND CYTOGENETIC ANALYSIS OF LAMBDA-20P7 FRAGMENT DNA FROMTHE PROXIMAL BETA-HETEROCHROMATIN OF DROSOPHILA-MELANOGASTER

A DNA fragment from the Drosophila melanogaster genome, cloned in lamb da 20p7, was derived independently from clones lambda 20 and lambda L [Baiborodin et al., Genetika 29 (1993) 403-416; Sharakhov et al., Gene tika 29 (1993) 392-402]. In situ hybridization of lambda 20p7 DNA to t he chromosomes of D. melanogaster demonstrated preferential hybridizat ion of the fragment to the chromocenter of polytene chromosomes and to pericentric heterochromatin of chromosomes II, IV and X at the metaph ase plate. Copy number per haploid genome for lambda 20p7 was estimate d as approximately 200. Based on Southern blotting, the major portion of this moderate repeat was localized in the region of a 5.5-kb HindII I digest. In situ hybridization to polytene chromosomes from strain fs (2)B trophocytes revealed that repeats homologous to lambda 20p7 are l ocated in the proximal heterochromatin which undergoes structural reor ganization during tissue differentiation. The nucleotide sequence of t wo segments of the clone lambda 20p7, 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 SpI, and a set of inverted repeats. Clone Dm270 contains an open reading frame (ORF). The deduced amino acid (a a) sequence shares homology with the gag-like gene from type-I (R1) ri bosomal DNA insertion and may code for a polypeptide of 10 kDa. The Dm 270 sequence was found to contain two direct repeats showing homology to the human CENP-B box.