The accessibility of DNA to dimethylsulfate in complexes with recA protein

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recA protein coats DNA co-operatively to form filaments approximately 100 A thick, which in the presence of ATP, and more stably so in the presence of the non-hydrolyzable analog ATP gamma S, have a helical appearance with a deep cleft in the protein coat. This protein helix follows the DNA helix, to which it imparts a new helicity of 18.5 bp per turn of 97 A pitch. Here we test the accessibility of the DNA in the complex to modification by dimethylsulfate, and find that the complexed DNA is approximately 2-fold more reactive on the major groove side than it was in B-DNA (methylation of guanine N7), while it is protected approximately 2-fold on the minor groove side (methylation of adenine N3), suggesting that the protein coats the DNA along the minor groove. Furthermore, N3 of cytosine, a residue involved in base pairing, is found exposed in complexes with single strands as it is in naked single-stranded DNA, while it remains inaccessible in complexes with double strands, suggesting that the latter is not melted at this stage of the strand exchange reaction.
Original languageEnglish
Pages (from-to)2493-2498
Number of pages6
JournalEMBO Journal
Issue number8
Publication statusPublished - 1 Aug 1987


  • Alkylating Agents
  • Base Sequence
  • DNA, Bacterial
  • Methylation
  • Microscopy, Electron
  • Plasmids
  • Rec A Recombinases
  • Sulfuric Acid Esters
  • Sulfuric Acids


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