Presence and expression of hydrogenase specific C-terminal endopeptidases in cyanobacteria

We identified genes encoding putative cyanobacterial hydrogenase specific C-terminal endopeptidases in all analyzed cyanobacterial genomes. The genes are not part of any known hydrogenase related gene cluster. The derived amino acid sequences show only low similarity (28–41%) to the well-analyzed hydrogenase specific C-terminal endopeptidase HybD from Escherichia coli, the crystal structure of which is known. However, computational secondary and tertiary structure modeling revealed the presence of conserved structural patterns around the highly conserved active site. Gene expression analysis shows that the endopeptidase encoding genes are expressed under both nitrogen-fixing and non-nitrogen-fixing conditions.Anabaena PCC 7120 possesses two NiFe-hydrogenases and two hydrogenase specific C-terminal endopeptidases but only one set of hyp-genes. Thus, in contrast to the Hyp-proteins, the C-terminal endopeptidases are the only known hydrogenase maturation factors that are specific. Therefore, in accordance with previous nomenclature, we propose the gene names hoxW and hupW for the bidirectional and uptake hydrogenase processing endopeptidases, respectively. Due to their constitutive expression we expect that, at least in cyanobacteria, the endopeptidases take over multiple functions.Hydrogenases catalyze the simplest of all chemical reactions: the reduction of protons to molecular hydrogen or vice versa. Depending on the metal content of the active site hydrogenases are classified into Fe-, NiFe-, and metal-free hydrogenases [1]. Independent from the metal content, the enzymes are characterized as hydrogen uptake, bidirectional and hydrogen evolving hydrogenases, indicating their actual in vivo activity. A prominent and evolutionary old group of organisms possessing NiFe-hydrogenases are phototrophic cyanobacteria (formerly blue-green algae) [2]. All cyanobacteria investigated so far, express an uptake, a bidirectional or both NiFe-hydrogenases [2-6]. The uptake hydroge