ANAEROBIC TRANSCRIPTION ACTIVATION IN BACILLUS-SUBTILIS - IDENTIFICATION OF DISTINCT FNR-DEPENDENT AND FNR-INDEPENDENT REGULATORY MECHANISMS

Bacillus subtilis is able to grow anaerobically using alternative elec tron accepters, including nitrate or fumarate. We characterized an ope ron encoding the dissimilatory nitrate reductase subunits homologous t o the Escherichia coli narGHJI operon and the narK gene encoding a pro tein with nitrite extrusion activity, Downstream from narK and co-tran scribed with it a gene (fnr) encoding a protein homologous to E.coli F NR was found, Disruption of fnr abolished both nitrate and fumarate ut ilization as electron accepters and anaerobic induction of narK, Four putative FNR binding sites were found in B.subtilis sequences, The con sensus sequence, centred at position -41.5, is identical to the consen sus for the DNA site for E.coli CAP, Bs-FNR contained a four cysteine residue cluster at its C-terminal end. This is in contrast to Ec-FNR, where a similar cluster is present at the N-terminal end, It is possib le that oxygen modulates the activity of both activators by a similar mechanism involving iron, Unlike in E.coli, where fnr expression is we akly repressed by anaerobiosis, fnr gene expression in B.subtilis is s trongly activated by anaerobiosis. We have identified in the narK-fnr intergenic region a promoter activated by anaerobiosis independently o f FNR. Thus induction of genes involved in anaerobic respiration requi res in B.subtilis at least two levels of regulation: activation of fnr transcription and activation of FNR to induce transcription of FNR-de pendent promoters.