We demonstrated previously that the promoters P1a, P1b, and P1c are very closely spaced and are coordinately turned on during stationary phase in Micromonospora echinospora. To determine the nucleotides important for promoter recognition, we characterized mutations that were defective in transcription from the P1b start site, using Streptomyces lividans as the host. Two mutations upstream of the start site resulted in a drastic loss of promoter activity, while two mutations downstream of the start site resulted in a moderate loss of activity. These mutations suggest an unexpected relationship between promoters P1b and P1c. Three of the mutations that caused diminished transcription from promoter P1b simultaneously resulted in an increase in transcription from the P1c promoter initiation site located 15 bp downstream. Despite the proximity and the coordinate regulation of promoters P1b and P1c, they are in competition as transcriptional start sites.
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