1. Departamento de Microbiologı́a II, Facultad de Farmacia, Universidad Complutense de Madrid, 28040 Madrid, Spain1
2. Departamento de Microbiologı́a y Genética, Instituto de Microbiologı́a-Bioquı́mica, Edificio Departamental, Campus Miguel de Unamuno, Universidad de Salamanca/CSIC, 37007 Salamanca, Spain2
3. Department of Molecular and Cell Biology, Division of Biochemistry and Molecular Biology, University of California, Berkeley, CA 94720, USA3