Effect of RecF protein on reactions catalyzed by RecA protein

RecF protein is one of at least three single strand DNA (ssDNA) binding proteins which act in recombination and repair in Escherichia coli. In this paper we show that our RecF protein preparation complexes with ssDNA so as to retard its electrophoretic movement in an agarose gel. The apparent stoichiometry of RecF-ssDNA-binding measured in this way is one RecF molecule for every 15 nucleotides and the binding appears to be cooperative. Interaction of the other two ssDNA-binding proteins, RecA and Ssb proteins, has been studied extensively; so in this paper we begin the study of the interaction of RecF and RecA proteins. We found that the RecF protein preparation inhibits the activity of RecA protein in the formation of joint molecules whether added before or after addition of RecA protein to ssDNA. It, therefore, differs from Ssb protein which stimulates joint molecule formation when added to ssDNA after RecA protein. We found that our RecF protein preparation inhibits two steps prior to joint molecule formation: RecA protein binding to ssDNA and coaggregate formation between ssDNA-RecA complexes and dsDNA. We found that it required a much higher ratio of RecF to RecA protein than normally occurs in vivo to inhibit joint molecule formation. The insight that these data give to the normal functioning of RecF protein is discussed.