Transcription of the Rhodobacter capsulatus nifHDK operon is modulated by the nitrogen source. Construction of plasmid expression vectors based on the nifHDK promoter.

We characterized the Rhodobacter capsulatus nifHDK promoter by nucleotide sequencing and nuclease S1 analysis of mRNA-protected DNA probes. Comparison of this promoter to nifP and ntrP promoters from other species reveals extensive homology to the canonical nifP consensus sequence. Using lac fusions we have demonstrated that transcription of the nifHDK operon is totally repressed when the growth medium is supplemented with ammonia, becomes fully derepressed in ammonia-free medium, and proceeds at intermediate levels when other nitrogen sources are used. Based on this information, we constructed plasmid expression vectors in which the rates of transcription from cloned DNA fragments are determined by the nitrogen source used in the growth medium.