1. Centro de Investigación sobre Fijación del Nitrógeno, Apartado Postal 565-A, Cuernavaca, Morelos, Mexico (R.A.-P.)
2. Department of Biochemistry and Biophysics, Iowa State University, Ames, Iowa 50011 (M.S.H.)
3. Department of Biochemistry, University of Nebraska, Lincoln, Nebraska 68588–0664 (J.F.M., G.S., R.V.K.)