Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): Fluorescence from an NtcA-green fluorescent protein (GFP) fusion after 12 h of nitrogen deprivation in Anabaena sp. strain PCC 7120, a filamentous cyanobacterium that develops nitrogen-fixing heterocysts when incubated in the absence of combined nitrogen. NtcA is the global transcriptional regulator of nitrogen assimilation genes in cyanobacteria, and the cells that exhibit a higher level of GFP fluorescence are proheterocysts. (See related article on page 6694.)