A DNA element recognised by the molybdenum-responsive transcription factor ModE is conserved in Proteobacteria, green sulphur bacteria and Archaea

Orthologues of ModE are widespread amongst diverse prokaryotes, but not ubiquitous. We identified probable ModE-binding sites upstream of genes implicated in molybdenum metabolism in green sulphur bacteria and methanogenic Archaea as well as in Proteobacteria. We also present evidence of horizontal transfer of nitrogen fixation genes between green sulphur bacteria and methanogenic Archaea.Whereas most of the archaeal helix-turn-helix-containing transcription factors belong to families that are Archaea-specific, ModE is unusual in that it is found in both Archaea and Bacteria. Moreover, its cognate upstream DNA recognition sequence is also conserved between Archaea and Bacteria, despite the fundamental differences in their core transcription machinery. ModE is the third example of a transcriptional regulator with a binding signal that is conserved in Bacteria and Archaea.The transition metal molybdenum is essential for life on earth. It is at the catalytic centre of over 30 enzymes, which are involved in the nitrogen, carbon, and sulphur cycles [1]. Molybdenum is found in the nitrogenase complex, which fixes dinitrogen gas, and in nitrate reductase, which reduces nitrate to nitrite. Other molybdo-proteins include xanthine oxidase, aldehyde oxidase, formate dehydrogenase, sulphite oxidase, nitrite reductase, DMSO reductase, pyridoxal dehydrogenase, xanthine dehydrogenase, and pyrogallol transhydrolase.Molybdenum is available to organisms in the form of the tetraoxyanion molybdate, which is transported into Escherichia coli cells by an ABC-type transport system encoded by the modABCD operon [2]. A ModE-molybdate complex binds specific DNA target sequences and thus represses or activates transcription of several operons in response to molybdate concentration. The ModE protein from E. coli consists of two domains [3]. At the N terminus is the DNA-binding domain (Pfam accession PF02573, HTH_9) containing a winged helix-turn-helix (HTH) motif. At the C-terminal end is the