Transcriptional regulation of a hybrid cluster (prismane) protein.

HCP (hybrid-cluster protein) contains two Fe/S clusters, one of which is a hybrid [4Fe-2S-2O] cluster. Despite intensive study, its physiological function has not been reported. The Escherichia coli hcp gene is located in a two-gene operon with hcr, which encodes an NADH-dependent HCP reductase. E. coli HCP is detected after anaerobic growth with nitrate or nitrite: possible roles for it in hydroxylamine or nitric oxide reduction have been proposed. To study the regulation and role of HCP, an hcp::lacZ fusion was constructed and transformed into fnr, arcA and norR mutant strains of E. coli. Transcription from the hcp promoter was induced during anaerobic growth. Only the fnr mutant was defective in hcp expression. Nitrate- and nitrite-induced transcription from the hcp promoter was activated by the response regulator proteins NarL and NarP. Gel retardation assays were used to show that FNR (fumarate-nitrate regulation) and NarL form a complex with the hcp promoter. Transcription of the hcp-hcr operon initiates at a thymine nucleotide located 31 bp upstream of the translation-initiation codon. HCP has been overexpressed from a recombinant plasmid for physiological studies.