Cover photograph (Copyright © 1999, American Society for Microbiology. All rights reserved.): Crystal structure of the Escherichia coli RuvA protein bound to Holliday junction DNA. A tetramer of RuvA (alternate subunits shown in white and grey) binds a Holliday junction recombination intermediate (shown in blue and yellow) via channels on the concave surface of the protein. Two conserved acidic residues (red) form the apex of a protruding pin-like structure which may facilitate strand separation during branch migration or ensure specific binding to a Holliday junction. The figure was produced with the RasMol program. (See related article on page 5543.)