The goal of this research project is to understand how the recA protein of Escherichia coli utilizes the energy of ATP hydrolysis to promote DNA strand pairing reactions during homologous genetic recombination and recombinational DNA repair. The major mechanistic principle to emerge from the work carried out during the previous funding period is that nucleoside triphosphate hydrolysis is not, in itself, sufficient to drive the strand exchange process; a nucleoside triphosphate must also be able to stabilize the polymeric recA protein-ssDNA complex in a strand exchange-active conformational state. The research described in this continuation application will investigate the allosteric properties of the recA protein-ssDNA complex and the role of the ATP-dependent conformational changes of the recA protein on the strand exchange reaction pathway. The specific goals are: 1. To develop a kinetic mechanism for the ATP-dependent isomerization of the recA-ssDNA complex. 2. To analyze the active site interactions between the recA protein and ATP which lead to the conformational changes of the recA protein. 3. To identify the regions of the recA protein that are involved in the binding of single stranded and duplex DNA. 4. To evaluate the contribution of subunit-subunit interaCtions to the function and conformational stability of the recA protein. These studies will increase our understanding of the molecular mechanisms of genetic recombination, as well as the mechanistic principles that are involved in energy transduction in macromolecular protein assemblies.