Endopeptidases belonging to F420-reducing hydrogenases group. These hydrogenases from methanogens are encoded by the fru, frc, or frh genes. Sequence comparison indicates that fruD and frcD gene products from Methanococcus voltae are similar to HycI protease of Escherichia coli and are putatively involved in the C-terminal processing of large subunits (FruA and FrcA respectively). FrhD (F420 reducing hydrogenase delta subunit) enzyme belongs to the gene cluster of 8-hydroxy-5-deazaflavin (F420) reducing hydrogenase (FRH) from the thermophilic methanogen Methanobacterium thermoautotrophicum delta H. FrhD subunit is putatively involved in the processing of the coenzyme F420 hydrogenase-processing. It is similar to those frhD genes found in Methanomicrobia and Methanobacteria. It is different from the FrhD conserved domain found in methyl viologen-reducing hydrogenase and F420-non-reducing hydrogenase iron-sulfur subunit D.