title: Base-unpaired regions in supercoiled replicative form DNA of coliphage M13.
abstract: Superhelical covalently closed circular replicative form DNA (RF I) of coliphage M13 appears as a relaxed molecule that has a base-unpaired region in the form of a bubble (100 to 200 base pairs long) seen in electron micrographs when spread in the presence of formaldehyde and formamide or after pretreatment with glyoxal. S1 endonuclease, specific for single-stranded DNA, converts superhelical M13 RF I DNA, but not nonsuperhelical M13 RF I to a significant extent, into unit-length linear molecules by sequential nicking of two strands. The locations of S1 nuclease-susceptible sites and glyoxal-fixed base-unpaired regions were both related to the five A-T-rich regions in M13 RF DNA. While S1 nuclease does not show preference for any of these sites, glyoxal-fixed bubbles occur predominantly at the major A-T-rich region in M13 RF DNA.
mesh_terms: D003090:Coliphages / Q000378:metabolism*; D004261:DNA Replication; D004270:DNA, Circular / Q000378:metabolism; D004279:DNA, Viral / Q000378:metabolism; D003851:Deoxyribonucleases; D004720:Endonucleases; D004926:Escherichia coli / Q000378:metabolism; D008854:Microscopy, Electron; D008970:Molecular Weight; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation
chemical_list: D004270:DNA, Circular; D004279:DNA, Viral; D003851:Deoxyribonucleases; D004720:Endonucleases