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The bld mutants of Streptomyces coelicolorA3(2) are blocked at the earliest stage of sporulation, the formation of aerial hyphae, and are pleiotropically defective in antibiotic production. Using a phage library of wild-type S. coelicolor DNA, we isolated a recombinant phage which restored both sporulation and antibiotic production to strains carrying the single known bldD mutation. Nucleotide sequence analysis of a 1.3-kb complementing subclone identified an open reading frame, designated bldD, encoding a translation product of 167 amino acid residues. Nucleotide sequence analysis of thebldD-containing fragment amplified from the chromosome of abldD mutant strain revealed a point mutation changing a tyrosine residue at amino acid position 62 to a cysteine. Although a comparison of the BldD sequence to known proteins in the databases failed to show any strong similarities, analysis of the BldD sequence …