1. Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039–0628, USA.
2. National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USA.
3. Molecular Dynamics, Amersham Pharmacia Biotech, Sunnyvale, CA 94086, USA.
4. Department of Microbiology and Molecular Genetics, The University of Texas Medical School, Houston, TX 77030, USA.