Replication protein A (RPA), the eukaryotic ssDNA binding protein, is a heterotrimeric protein required for almost all aspects of cellular DNA metabolism. RPA binds single-stranded DNA with high affinity. We want to elucidate the roles of RPA in DNA replication, using integrated structural and biophysical methods, using the Archaeon Pyrococcus abyssi as a model organism. RPA is an ideal target for small angle scattering studies as there is partial structural information available on the individual protein subunits, an incomplete structure available of the trimerization core, and of partial structures of portions of RPA bound to DNA. We will carry out SANS to determine the solution structures of each RPA protein component, contrast variation SANS experiments to obtain the structure of the two possible protein dimers, the full RPA trimer structure, as well as the complexes with DNA.