Cover photograph (Copyright © 2007, American Society for Microbiology. All Rights Reserved.): Atomic force microscopy (AFM) images of individual molecules of chromatin showing compaction by the nucleosome-binding enzyme poly(ADP-ribose) polymerase 1 (PARP-1). Chromatin was assembled in vitro using an ∼10.5-kb circular plasmid. The images shown are chromatin alone (top), chromatin after the addition of PARP-1 (middle), and chromatin after the sequential addition of PARP-1 and NAD⁺ (bottom). The addition of NAD⁺ promotes the auto(ADP-ribosyl)ation of PARP-1, the release of PARP-1 from nucleosomes, and the decondensation of chromatin. The AFM images were obtained in the amplitude mode. (See related article on p. 7475.) See erratum.