Cover photograph (Copyright © 2003, American Society for Microbiology. All Rights Reserved.): Whereas Streptomyces coelicolor colonies normally produce white aerial hyphae that differentiate into spores, this mutant colony remains smooth and shiny while overproducing a red-pigmented antibiotic. Its defect in colonial differentiation is due to inactivation of adpA, an araC-type transcriptional regulatory gene. Within adpA is a unique TTA triplet that requires the corresponding leucyl-tRNA gene, bldA. This partially explains the dependency of aerial mycelium formation on bldA, which is believed to have developmental control functions. (See related article on page 7291.)