Transcriptional regulation of the four promoters of the agarase gene (dagA) of Streptomyces coelicolor A3(2).

The agarase gene (dagA) of Streptomyces coelicolor A3(2) is transcribed from four promoters that are recognized by at least three, and probably four, different RNA polymerase holoenzymes, each containing a different sigma factor. S1 nuclease protection studies revealed that transcription from all four promoters is induced by the products of agar hydrolysis and strongly repressed by glucose. Mutants deficient in glucose kinase activity were defective in glucose repression of all four promoters. Mutants were isolated or identified in which transcription from all four promoters had become inducer-independent (i.e. constitutive), establishing the existence of a repressor gene for dagA that does not appear to be located within 9 kb of the structural gene. The cloned dagA gene was also constitutively expressed in the closely related strain Streptomyces lividans, which does not normally make agarase and which presumably lacks the repressor gene. Glucose was still able to repress dagA transcription even under conditions of constitutive expression, suggesting that glucose kinase does not mediate its effect via inducer exclusion. Relative differences in the use of the four promoters were not detected during different stages of growth of surface-grown cultures, although dagA transcription appeared to peak during the production of aerial mycelium.