1. Departamento de Microbiologı́a Molecular, Instituto de Biotecnologı́a, Universidad Nacional Autónoma de México, Cuernavaca, Morelos 62250, Mexico1;
2. Department of Microbiology and Cell Science, University of Florida, Gainesville, Florida 326112;
3. Department of Chemistry, City College of New York, New York, New York 100313; and
4. BioScience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 875444