Department of Biochemistry and Molecular Biology A110, Life Science Building, 120 Green Street, University of Georgia, Athens, GA 30602-7229, USA. Tel.: +706-542-9779, Fax: +706-542-9751/7782, xyn@bmb.uga.edu, Zhiwei Cao, School of Life Sciences and Technology, Tongji University, 1239 Siping Road, Shanghai 200092, China. Tel.: +86-21-54065003, Fax: +86-21-65980296, E-mail: zwcao@tongji.edu.cn.