TheStreptomyces coelicolorDevelopmental Transcription Factor σBldNIs Synthesized as a Proprotein

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Abstract
bldN is one of a set of genes required for the formation of specialized, spore-bearing aerial hyphae during differentiation in the mycelial bacterium Streptomyces coelicolor. Previous analysis (M. J. Bibb et al., J. Bacteriol. 182:4606-4616, 2000) showed that bldN encodes a member of the extracytoplasmic function subfamily of RNA polymerase σ factors and that translation from the most strongly predicted start codon (GTG1) would give rise to a σ factor having an unusual N-terminal extension of ca. 86 residues. Here, by using a combination of site-directed mutagenesis and immunoblot analysis, we provide evidence that all bldN translation arises from initiation at GTG1 and that the primary translation product is a proprotein (pro-σBldN) that is proteolytically processed to a mature species (σBldN) by removal of most of the unusual N-terminal extension. A time course taken during differentiation of the wild type on solid medium showed early production of pro-σBldN and the subsequent appearance of mature σBldN, which was concomitant with aerial mycelium formation and the disappearance of pro-σBldN. Two genes encoding members of a family of metalloproteases that are involved in the regulated proteolytic processing of transcription factors in other organisms were identified in the S. coelicolor genome, but their disruption did not affect differentiation or pro-σBldN processing.