1. Department of Microbiology and Cell Science, University of Florida, Gainesville, FL 32611, USA
2. Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
3. Present address: Department of Biochemistry, School of Medicine, Vanderbilt University, Nashville, TN 37232, USA
4. Department of Chemistry, University of Florida, Gainesville, FL 32611, USA
5. Present address: Department of Biology, Haverford College, 370 Lancaster Ave., Haverford, PA, USA
6. Division of Biological Sciences, University of Montana, Missoula, Montana, MT 59812, USA
7. Genetic Institute, University of Florida, FL 32611, USA