title: Direct observation of complexes of ssb and recA proteins with a fluorescent single-stranded deoxyribonucleic acid derivative.
abstract: Evidence is presented from fluorescence and kinetic experiments that ssb binds to a recA-ssDNA-ATP complex causing a major structural change in which some 40% of the bound recA is released. On addition of ssb to recA-epsilon DNA-ATP (containing the fluorescent analogue of ssDNA epsilon DNA), there is a slow first-order decrease in fluorescence (t1/2 approximately 3 min). This is accompanied by a loss in the ATPase activity of recA protein. The resultant complex does not exchange epsilon DNA for added ssDNA. Measurement of the DNA-stimulated ATPase activity on addition of excess ssDNA reveals that 40% of the previously bound recA has been released. The stoichiometry of recA bound to epsilon DNA thus changes from 1 mol per six nucleotides to 1 per 10 on addition of ssb. Formation of the ssb-recA-epsilon DNA complex is dependent on ATP, and the rate varies with the concentration of ssb.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D004277:DNA, Single-Stranded / Q000031:analogs & derivatives* / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005453:Fluorescence; D008274:Magnesium / Q000494:pharmacology; D009994:Osmolar Concentration; D011938:Rec A Recombinases / Q000378:metabolism*
chemical_list: D004277:DNA, Single-Stranded; D004268:DNA-Binding Proteins; D011938:Rec A Recombinases; D000251:Adenosine Triphosphatases; D008274:Magnesium