title: Topologically constrained domains of supercoiled DNA in eukaryotic cells.
abstract: The size of supercoiled, topologically constrained DNA domains within the squamous carcinoma cell line SQ-20B were determined by direct comparison with a panel of irradiated supercoiled plasmid DNAs. Loss of supercoiling in plasmids was determined by gel electrophoresis and in cells by nucleoid flow cytometry. Comparison of dose-response data for plasmid relaxation with that obtained from SQ-20B cells enabled a direct estimation of supercoil target size in these cells. Plasmids pUCD9P (3.9 kbp), pXT-1 (10.1 kbp), pdBPV-MMT-neo (14.6 kbp), pRK290 (20.0 kbp), and R6K (38 kbp) were used and analyzed under the same exposure conditions as nucleoid DNA. Two sizes of topologically closed domains were found in nucleoids of 0.51+/-0.17Mbp and 1.34+/-0.3 Mbp. In an attempt to relate these large-scale organizations of DNA with function, cells were exposed to the DNA topoisomerase II inhibitor, VP16 and the G1/S cell cycle blocking agent mimosine. A 1 h exposure to VP16 was effective in reducing DNA synthesis which was associated with a parallel increase in nucleoid supercoiling. Addition of the G1 > S inhibitor mimosine enhanced both responses. It is concluded that chromosomes and interphase nuclei are organized into at least two sizes of topologically constrained domains of DNA which may have functional relevance to the control and execution of DNA synthesis.
authors.affiliation: Loyola University Medical Center, Dept. of Radiotherapy, Cancer Center, Maywood, IL 60153, USA.
authors.forename: N N
authors.initials: NN
authors.lastname: Khodarev
authors.forename: A
authors.initials: A
authors.lastname: Narayana
authors.forename: A
authors.initials: A
authors.lastname: Constantinou
authors.forename: A T
authors.initials: AT
authors.lastname: Vaughan
mesh_terms: D002294:Carcinoma, Squamous Cell; D004247:DNA / Q000096:biosynthesis; D004278:DNA, Superhelical / Q000737:chemistry*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016193:G1 Phase / Q000187:drug effects; D005720:Gamma Rays; D017405:Herpes Simplex Virus Protein Vmw65 / Q000494:pharmacology; D006801:Humans; D008898:Mimosine / Q000494:pharmacology; D009690:Nucleic Acid Conformation / Q000528:radiation effects; D010957:Plasmids / Q000737:chemistry / Q000528:radiation effects; D059005:Topoisomerase II Inhibitors; D014407:Tumor Cells, Cultured
chemical_list: D004278:DNA, Superhelical; D004791:Enzyme Inhibitors; D017405:Herpes Simplex Virus Protein Vmw65; D059005:Topoisomerase II Inhibitors; D008898:Mimosine; D004247:DNA