Properties of RecA‐oligonucleotide complexes

The interaction of RecA protein with short single‐stranded oligonucleotides is characterised by flow linear dichroism (LD), isoelectric focusing (IEF) and electron microscopy (EM). From LD and EM it is evident that RecA forms long filaments with at least some 50 oligonucleotides in a ‘train formation’. The tendency to form trains is substantially lower when an amino group is attached to the 5′ end of the oligonucleotide, suggesting that the modification impairs protein‐protein interactions at the interface between two oligomers. From LD it is also evident that no bridging occurs between RecA–Oligonucleotide complexes containing more than one oligomer strand per RecA filament. This property make them manageable in polyacrylamide gels, hence allowing characterisation by IEF. RecA was found acidic with a p I of 5.0. The p I was not dependent on the presence of bound cofactor (ATPγS) and oligonucleotides suggesting that protonation of the protein readily occurs to compensate for the negative charges provided by bound cofactor and DNA.