DNA bending induced by the catabolite activator protein allows ring formation of a 144 bp DNA

The effect of the catabolite activator protein, CAP, on the ligation of a 144 bp DNA was examined. This DNA has EcoRI ends and contains the lac operon CAP site and promoter-operator region. At low DNA concentrations (nM) and 37 degrees C the presence of CAP and cAMP enables T4 ligase to convert the linear duplex to a covalently closed ring. Nuclease digestion and sedimentation equilibrium studies show that the ring is a monomer circle. Ring formation does not occur in the absence of either CAP or cAMP. The kinetics of ring closure, and the bimolecular joining of two fragments were measured. The presence of CAP decreased the rate of bimolecular joining of the EcoRI ends of linear DNAs. Thus the measured rates of ring closure are likely to be a lower limit for this process. Closure reactions carried out with ethidium bromide indicate that CAP induced bending rather than twisting is responsible for ring formation. The all or none nature of the closure reaction suggests that persistence length DNAs may be useful in a simple assay for protein induced DNA bending.