ACTIVATION OF RECA PROTEIN - THE PITCH OF THE HELICAL COMPLEX WITH SINGLE-STRANDED-DNA

The complex of recA protein with single-stranded DNA in the presence of ATP is the active species in the three enzymatic activities of recA: the initiation of strand exchange, the hydrolysis of ATP and the cleavage of repressors. Here we find by cryo-electron microscopy of unstained and unfixed samples that the helical structure of the protein coat in this complex differs slightly but significantly from the structure in the complex with double-stranded DNA. We discuss how the larger pitch of the complex with single strands (100 +/- 2 angstrom compared with 95 +/- 2 angstrom with double strands) could contribute to its higher enzymatic activity.