Transcriptional and translational features of a sporulation gene of Streptomyces griseus.

The nucleotide (nt) sequence of a 2.8-kb fragment of DNA that restores sporulation to one class of bald mutants of Streptomyces griseus revealed an open reading frame (ORF) with the potential to encode a 55.5-kDa polypeptide. The presence of an in-frame TTA in the coding sequence indicated that translation is likely to require the tRNA(Leu)UUA encoded by the bldA gene. Two overlapping transcripts are initiated at transcriptional start points (tsp) separated by 258 nt and are transcribed in the same direction. The downstream tsp lies within the ORF and is followed by a second potential translation initiation site, which would encode a 49.5-kDa polypeptide in the same reading frame as the 55.5-kDa polypeptide. Transcription assays suggested that both tsp functioned during vegetative growth, but the relative abundance of the shorter transcript decreased during the early stages of submerged sporulation. Analysis of sequentially deleted subclones indicated that expression of the longer ORF was necessary to complement bald mutants. The presence of two tsp alternating with potential translation start codons suggests the temporally regulated synthesis of two polypeptides that have identical C termini but different N termini.