Polytene chromosome interband DNA is organized into nucleosomes
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O R I GI N A L P A P E R
Yu. B. Schwartz á S. A. Demakov á I. F. Zhimulev
Polytene chromosome interband DNA is organized into nucleosomes
Received: 11 February 2000 / Accepted: 31 October 2000 / Published online: 19 January 2001 Ó Springer-Verlag 2001
Abstract The molecular basis that underlies the maintenance of polytene chromosome banding pattern remains unclear. To test the possibility that the decondensed state of interbands is provoked by the absence of nucleosomes, we have subjected chromatin from the previously de®ned 61C7/C8 interband to digestion with micrococcal nuclease. We have demonstrated that interband DNA forms nucleosomes both in salivary glands and in the bulk of larval tissues. This ®nding strongly suggests that the dierence in compaction between DNA in polytene chromosome bands and interbands results from dierences that appear at the higher levels of chromatin organization. Key words Polytene chromosome á Interbands á Nucleosomes á Micrococcal nuclease
Introduction Because of the clear morphology arising from the alternation of densely packed stretches of chromatin (bands) and less tightly packed regions (interbands), the polytene chromosomes of Diptera have become a most fruitful model for studying the relationship between changes in structure and the functional state of the interphase chromosome (for detailed reviews, see Zhimulev 1996; 1999). However, the molecular basis of the polytene chromosome banding pattern formation still remains unclear. Surprising as it may seem, it is not exactly known what accounts for the dierence in DNA packing density between band and interband. It has been well documented that bands contain 20±30 times Communicated by D. Gubb Yu. B. Schwartz á S. A. Demakov á I. F. Zhimulev (&) Institute of Cytology and Genetics, Russian Academy of Sciences, Lavrent'ev Ave. 10, 630090 Novosibirsk, Russia E-mail: [email protected] Fax: +7-3832-331278
more DNA than interbands (Paul and Mateyko 1970; Sorsa 1982; Kozlova et al. 1994). In interphase nuclei most of the DNA is wrapped around histone octamers, forming the 10 nm diameter nucleosome ®bril in which DNA is compacted by a factor of about 5 (reviewed by Kornberg and Lorch 1999). The 10-nm nucleosome ®lament is further folded into the 30-nm nucleomere ®bril in which DNA is approximately 25-fold compressed (Finch and Klug 1976; Bednar et al. 1998). Finally the 30-nm ®bril is believed to be packed in some kind of higher-order structures (Laemmli et al. 1992; Razin 1996). The dierence in chromatin compaction between bands and interbands may arise at any of the stages of DNA folding mentioned above. As a starting point it is important to determine whether the nucleosome organization (i.e. 10-nm ®bril) persists in both bands and interbands. To address this question several experiments were performed in the 1980s, the results, however, appeared to be controversial. Electron microscopic (EM) observations of interband regions in whole-mount polytene chromosomes from the salivary glands of ®rst- and second-instar larvae
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