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M. Pierre and . Curie, ARECA), and the European Communities (project QLG1-CT1999-00202). R. L. was supported by an ARC fellowship and I. I, by a fellowship from the Ministère délégué à la Recherche et aux Technologies Nouvelles (MRT) and the Université Pierre et Marie Curie. 2 Equally contributed to this work 3 Correspondence address: Dr. Marc Daëron

F. Paris and . Tel, +33-1-4568-8642, Fax: +33-1-4061-3160, E-mail: daeron@pasteur