1URA-CNRS 1446, Université Louis Pasteur 21, rue René Descartes, 67 084 Strasbourg Cedex, France
2Corresponding Author: Marie-José Freund-Mercier
ACKNOWLEDGEMENTS: This work was supported by the Centre National de la Recherche Scientifique (CNRS-URA 1446). The authors are grateful to E. Waltisperger, M.J. Klein and J.M. Gachon for expert technical assistance.
Received 20 June 1997; accepted 13 July 1997