[1] Montana State Univ, Dept Microbiol & Immunol, Bozeman, MT 59717 USA
[2] Montana State Univ, Dept Chem & Biochem, Bozeman, MT 59717 USA
[3] Montana State Univ, Dept Plant Sci & Plant Pathol, Bozeman, MT 59717 USA
[4] Max Planck Inst Biochem, Dept Mol Struct Biol, D-82152 Martinsried, Germany