Supragenic loop organization: mapping inDrosophilaembryos, of scaffold-associated regions on a 800 kilobase DNA continuum cloned from the 14B-15B first chromosome region

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Abstract
The supragenic loop organization of the Drosophila genome was investigated on a 800 kilobase (kb) DNA continuum from the 14B-15B first chromosome region. Nuclear scaffolds from 0-18 hr embryos were prepared with Laemmli''s low-salt, detergent procedure and digested with restriction enzymes. Scaffold-associated regions (SARs) were mapped by probing Southern transfers of total, scaffold-associated and free DNA with a set of 70 recombinant phages overlapping the investigated genomic region. In all, 85 restriction fragments showed association to scaffolds. 12 of them were present in the majority of scaffolds. They bore strong SARs organizing the DNA molecule as consecutive loops with sizes ranging from 15 to 115 kb. 44 were present in only a fraction of scaffolds. They contained weak SARs subdividing the basic loops into smaller ones. 29 additional restriction fragments were present in a very small fraction of scaffolds. The position of SARs with respect to transcribed regions was investigated. Strong SARs appeared to be located on untranscribed DNA and to frame transcription units. In contrast, at least some weak SARs were shown to comap with transcribed regions or to reside within characterized transcription units. Statistical analyses established that strong and weak SARs were periodically positioned on the DNA continuum and that there was a potential contact point between scaffolds and the DNA continuum every 11 kb, or multiples thereof. Implications for SAR role(s) are discussed.