1. G. J. Olsen, C. I. Reich, B. P. Kaine, and C. R. Woese are in the Microbiology Department, University of Illinois, Champaign-Urbana, IL 61801, USA.
2. R. Overbeek is with the Division of Mathematics and Computer Science, Argonne National Laboratory, Argonne, IL 60439, USA.
3. J. M. Merrick is in the Department of Microbiology, State University of New York, Buffalo, NY 14214, USA.
4. M. Borodovsky is with the School of Biology, Georgia Institute of Technology, Atlanta, GA 30332, USA.
5. H. O. Smith is in the Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA. All other authors are with The Institute for Genomic Research (TIGR), Rockville, MD 20850, USA.