1. Department of Microbiology; University of Washington; Seattle WA 98195 USA
2. Department of Molecular, Cellular and Developmental Biology; University of California, Santa Barbara; Santa Barbara CA 93106 USA
3. Department of Pharmaceutical Sciences; School of Pharmacy; University of Maryland; Baltimore MD 21201 USA
4. Biomolecular Science and Engineering Program; University of California, Santa Barbara; Santa Barbara CA 93106 USA