Summary: growing under anaerobic conditions produces several molybdoenzymes, such as formate hydrogenlyase (formate to H and CO; and genes) and nitrate reductase ( genes). Synthesis of these molybdoenzymes, even in the presence of the cognate transcriptional activators and effectors, requires molybdate in the medium. Besides the need for molybdopterin cofactor synthesis, molybdate is also required for transcription of the genes encoding these molybdoenzymes. In ModE was previously identified as a repressor controlling transcription of the operon encoding molybdate transport components (). In this work, the ModE protein was also found to be a required component in the activation of to an optimum level, but only in the presence of molybdate. Mutant ModE proteins which are molybdate-independent for repression of also restored expression to the wild-type level even in the absence of molybdate. Nitrate-dependent enhancement of transcription of was completely abolished in a mutant. Nitrate-response by and was reduced by about 50% in a mutant. DNase I footprinting experiments revealed that the ModE protein binds the promoter DNA in the presence of molybdate. ModE-molybdate also protected DNA in the intergenic region between and from DNase I hydrolysis. DNA sequences (5′ TAYAT 3′ and 5′ GTTA 3′) found in ModE-molybdate-protected operator DNA were also found in the ModE-molybdate-protected region of promoter DNA (5′ GTTA-7 bp-CATAT 3′) and intergenic region (5′ GTTA-7 bp-TACAT 3′). Based on these results, a working model is proposed in which ModE-molybdate serves as a secondary transcriptional activator of both the and operons which are activated primarily by the transcriptional activators, FhIA and NarL, respectively.


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