Chromatin structure and extrachromosomal circular DNA of tandemly repeated genomic sequences in Drosophila. D. Segal, S. Cohen, K. Bronstein, N. Agmon. Molecular Microbiol & Biotech, Tel Aviv Univ, Tel Aviv, Israel.

   One characteristic of genomic plasticity is the presence of extrachromosomal circular DNA (eccDNA). This DNA is found in various eukaryotes from yeast to humans. It is heterogeneous in size and composed of chromosomal sequences. Elevated levels of eccDNA are correlated with genomic instability and exposure to carcinogens. We used 2-dimensional gel electrophoresis to detect and characterize eccDNA in Drosophila. Recently we reported that eccDNA is detected throughout the fly's life cycle. These molecules comprise up to 10% of the total repetitive DNA content and their size ranges from <1kb to >20kb. The eccDNA population contains circular multimers of tandemly repeated genes such as histones, rDNA, Stellate and the Suppressor of Stellate. Centromeric heterochromatin sequences such as the dodeca satellite, and the satellites I (1.672 g/cm³) and III (1.688 g/cm³, 359 bp repeat) are included in eccDNA as well. Hence, any tested genomic tandem repeat was represented in eccDNA. However, the relative amount of eccDNA derived from different repeats is not the same. This suggests that different tandem repates are not equally prone to formation of eccDNA. This finding and the fact that most of the genomic tandem repeats are heterochromatic, imply a possible role of chromatin structure in the formation of eccDNA. To address this issue, we test the effect of several chromatin-modifying genes on eccDNA. Amongst the tested genes are Su(var)2-5, Su(var)3-9, dsir2, D1, abo and HDAC1. Data describing the effect of mutants of these genes on the amount and sequence content of eccDNA will be presented.