1. Department of Microbiology and Immunology, Hearst Microbiology Research Center and Strang Cancer Prevention Center, The Joan and Sanford I. Weill Medical College of Cornell University, 1300 York Avenue, Box 62, New York, New York 10021, and Department of Biological Sciences, South Carolina Experiment Station, Clemson University, 132 Long Hall, Clemson, South Carolina 29634-0326