1. Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803; Departamento de Bioquímica and Centro de Neurociências e Biologia Celular and Departamento de Zoologia, Universidade de Coimbra, 3004-517 Coimbra, Portugal; and Institut für Angewandte Mikrobiologie, Justus-Liebig-Universität Giessen, D-35392 Giessen, Germany