Cover photograph (Copyright © 2005, American Society for Microbiology. All Rights Reserved.): Structure of Escherichia coli lactaldehyde:propanediol oxidoreductase. Illustration shows a ribbon diagram color coded from the γ/β N-terminal domain (blue-green) to the all γ-helical C-terminal domain (yellowish-red). A deep cleft between these domains defines the active center, where the catalytic iron (purple) and the coenzyme NAD⁺ (stick-and-ball structure) are lodged. This enzyme has been used extensively by E. C. C. Lin and collaborators as a model in the experimental study of the evolution of metabolic function. (See related article on page 4957).