Cover photograph (Copyright © 2015, American Society for Microbiology. All Rights Reserved.): Structural detail of the substrate binding site of the HD-GYP protein PA4781 from Pseudomonas aeruginosa, showing two octahedrally coordinated metal ions. The digital artwork was obtained by overlaying the silhouette of the structure on a watercolor background. (Image courtesy of Giorgio Giardina.) (See related article on page 1525.)