The accessibility of DNA to dimethylsulfate in complexes with recA protein.
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.
Di Capua
E
E
Institute of Cell Biology, ETH Hönggerberg, Zürich, Switzerland.
Müller
B
B
D000477:Alkylating Agents; D001483:Base Sequence; D004269:DNA, Bacterial / Q000378:metabolism* / Q000648:ultrastructure; D008745:Methylation; D008854:Microscopy, Electron; D010957:Plasmids; D011938:Rec A Recombinases / Q000378:metabolism*; D013463:Sulfuric Acid Esters; D013464:Sulfuric Acids
D000477:Alkylating Agents; D004269:DNA, Bacterial; D013463:Sulfuric Acid Esters; D013464:Sulfuric Acids; D011938:Rec A Recombinases; C007482:dimethyl sulfate