1. Department of Genetics and Development, University of Illinois, Urbana 61801, USA.
2. Christian-Albrechts-Universitat, Olshausenstr. 40160, D-23 Kiel, Federal German Republic
3. Lehrstuhl fur Mikrobiologie, Technische Universitat Munchen, Arcisstr. 16, D-8000 Munchen 2, Federal German Republic