editor in chief, Nancy L. Craig, Howard Hughes Medical Institute, Department of Molecular Biology & Genetics, Johns Hopkins University School of Medicine, Baltimore, MD ; editors, Michael Chandler, Laboratoire de Microbiologie et Génétique Moléculaire, C.N.R.S., F-31062 Toulouse Cedex, France, Martin Gellert, National Institutes of Health, Molecular Genetics Section, NIDDK, Bethesda, MD, Alan M. Lambowitz, Institute for Cellular Molecular Biology, and Department of Molecular Biosciences, University of Texas at Austin, Austin, TX, Phoebe A. Rice, Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, IL, Suzanne B. Sandmeyer, Departments of Biological Chemistry and Microbiology and Molecular Genetics, University of California, Irvine, CA