1. Department of Physiology and Biophysics, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461, Department of Microbiology and Molecular Genetics, University of Texas Medical School, Health Science Center, 6431 Fannin, Houston, Texas 77030, and Department of Microbiology, EC Slater Institute, University of Amsterdam, Nieuwe Achtergracht 127, 1018 WS Amsterdam, The Netherlands