Regulation of transcription of the Escherichia coli moe operon, which codes for proteins connecting molybdate metabolism, molybdopterin synthesis, and apomolybdoenzyme synthesis, was investigated. Expression of the moe operon was independent of genes coding for molybdate transport and Mo-cofactor biosynthesis. Expression of moeA-lacZ increased during anaerobic growth (2.5-fold over the aerobic value) and in the presence of nitrate and trimethylamine N-oxide (3.5- and 1.5-fold, respectively). The nitrate-dependent increase in moe expression required the NarL protein, while the anaerobiosis-dependent increase in moeA-lacZ expression required Arc proteins. ArcA-phosphate and not ArcA bound to the DNA upstream of moe, shifted the electrophoretic mobility of moe promoter DNA, and protected the DNA from DNase I hydrolysis. Nitrate-independent transcription of moeA-lacZ was repressed by the FNR protein, which also protected moe operator DNA from DNase I hydrolysis. These results show that ArcA-phosphate and FNR have opposite effects on the transcriptional regulation of the moe operon, and the combined action of the two redox regulators modulate the level of Mo-cofactor in the cell. Apparently, the control of synthesis of Mo-cofactor and the apomolybdoenzymes nitrate reductase and trimethylamine N-oxide reductase are coupled at the level of the moe operon.