Critical residues and novel effects of overexpression of the Streptomyces coelicolor developmental protein BldB: evidence for a critical interacting partner.

The bldB gene of Streptomyces coelicolor encodes the best-characterized member of a family of small proteins that have low isoelectric points but that lack any previously characterized sequence motifs. BldB is dimeric and is required for the efficient production of antibiotics and spore-forming cells, called aerial hyphae, by growing colonies. The mechanism of action of BldB and its relatives is unknown. Here, we have explored amino acids in BldB that either are highly conserved or have been implicated in function genetically. We show that five amino acids are important for its function at physiological expression levels. Mutations in three of these amino acids gave rise to proteins that were either monomeric or unstable in vivo, while two others are not. We find that overexpression of bldB in S. coelicolor blocks sporulation prior to sporulation-specific septation but permits the formation of aerial hyphae. Vegetative septation was apparently normal in both the bldB null mutant and the bldB overexpression strain. To our surprise, overexpression of the dimerization-competent but functionally defective alleles caused a dramatic acceleration of sporulation. Our results suggest that BldB makes at least one important contact with another subcellular constituent and that a loss or alteration of this interaction impairs the phenotypic properties of the organism.

Structural and genetic analysis of the BldB protein of Streptomyces coelicolor.

We have demonstrated that the bldB gene of Streptomyces coelicolor is required for the formation of aerial hyphae and the synthesis of antibiotics. We also found that BldB forms a higher-order complex (most likely a dimer) and that amino acid residues 20 to 78 are important for this interaction. This region is conserved in the BldB family, suggesting that dimer formation may be a common feature of these proteins.