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.