Cover picture: Ribbon representation of the structures of [4Fe-4S]-type ferredoxins from Peptostreptococcus asaccharolyticus (a), Chromatium vinosum (b), Azotobacter vinelandii (c), Bacillus schlegelii (d), and Desulfovibrio africanus (e). These ferredoxins all have an extra, non-ligating cysteine near one of their [4Fe-4S]-type clusters, which are shown in ball-and-stick representation with sulfurs (yellow), irons (magenta), and carbons (cyan). The presence of an extra cysteine versus an alanine at this position leads to a decrease in the reduction potential of the nearest cluster because of differences in the electrostatic environment. The major change is a shift in the polar backbone depending on whether a cysteine or an alanine is present, with an SH···S hydrogen bond between the cysteine side chain and the cluster stabilizing the cysteine conformation. See the article by Beck et al. on page 601.