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The periplasmic cytochrome c 550 content of Paracoccus denitrificans has been shown by immunological detection to be strongly dependent on the mode of growth. Cells grown under anaerobic, denitrifying conditions or methylotrophically in the presence of oxygen contained substantially more cytochrome c 550 than cells grown aerobically on multicarbon substrates. A similar pattern was observed when expression of the cycA gene (encoding cytochrome c 550), was monitored using an Escherichia coli alkaline phosphatase gene (phoA) fusion as a reporter of cycA promoter activity. The increase in cycA expression observed during growth on C1 substrates was substantially diminished if succinate was also present. These results reveal that expression of cycA is subject to multiple regulatory controls and suggest that cytochrome c 550 has a general role in electron transfer to periplasmic reductases required for anaerobic denitrifying growth and from dehydrogenases required for aerobic growth on C1 compounds. Two major transcriptional initiation start points for the cycA gene have been identified.
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