Abstract:Cyanobacteria are the only prokaryotes that are able to produce clean, renewable fuel hydrogen gas through oxygenic photosynthesis. The hox-encoded bidirectional Ni-Fe hydrogenases in some cyanobacteria species, capable of evolving hydrogen gas, has continued to attract interest due to their application potential. However, the regulation of their enzymatic activity remains largely unexplained. In this review, we summarized the structure, ecological distribution and expression regulation of Hox hydrogenases in cyanobacteria. Besides, we briefly introduced our recent studies on two hypothetic genes, ssl2420 and sll1225 in the hox operon of Synechocystis sp. PCC 6803, and discussed the possible roles of this gene pair in the differential expression of hox components.