Department of Biology, University of California at San Diego, La Jolla, CA, 92093-0116, , USA1
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA, 02115, , USA2
Author for correspondence: M. H. Saier, Jr. Tel: +1 858 534 4084. Fax: +1 858 534 7108. e-mail: msaier{at}ucsd.edu