Structural biology of a plasmid addiction system.

Plasmid addiction svstems are one of the tools developed by nature to ensure the stable maintenance of a plasmid in a bacterial population. In the case of the so-called "proteic" plasmid addiction system, the plasmid contains the genes of both a toxin and its antidote. During normal cell growth, both proteins are expressed. Thus, the antidote counteracts the toxin via the fonnation of a molecular complex. If after cell division one of the daughter cells looses is left without plasmid, the instable antidote will be degraded faster than the stable toxin. Our research project concerns the biochemistry and structural biology of such plasmid addiction systems, and of the ccd system in particular. In the case of the ccd addiction system of plasmid F, CcdB (the toxin) poisons the DNA-gyrase covalent intermediate, Nvith which it forms a stable One of the challenges of developmental biology is to understand how cells know their position in the embryo and respond to it by proliferating, migrating or differentiating, leading ultimately to an organism. This problem can be studied during gastrulation, the first major event in the formation of an embryo. The family of Wnt genes (about 20 members) which encode secreted proteins are involved in numerous signaling developmental processes in organisms such as the fly (i.e.Wingless), the worm as well as the vertebrates. One major problem in the study of the Wnt genes is that no soluble vertebrate Wnt protein has yet been obtained that would allow the analysis of its role in defined experimental system. Only recently,were we able to obtain active frog Wnt-8 protein (my own unpublished results).In this project, we propose to study the signaling and inductive properties of this newly produced Xenopus Wnt-8 protein on frog embryo explants and extend this approach to other Wnt
proteins that will be produced in the future.

complex. We have recently determined the crystal structure of CcdB (Loris et al. (1999) J. Mol. Bi