1. Center for Molecular Biophysics, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831, Department of Biochemistry and Cellular and Molecular Biology, University of Tennessee, M407 Walters Life Sciences 1414 Cumberland Avenue, Knoxville, Tennessee 37996, and BioEnergy Science Center, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831