eWe have thoroughly investigated the abrB2 gene (sll0822) encoding an AbrB-like regulator in the wild-type strain of the model cyanobacterium Synechocystis strain PCC6803. We report that abrB2 is expressed from an active but atypical promoter that possesses an extended ؊10 element (TGTAATAT) that compensates for the absence of a ؊35 box. Strengthening the biological significance of these data, we found that the occurrence of an extended ؊10 promoter box and the absence of a ؊35 element are two well-conserved features in abrB2 genes from other cyanobacteria. We also show that AbrB2 is an autorepressor that is dispensable to cell growth under standard laboratory conditions. Furthermore, we demonstrate that AbrB2 also represses the hox operon, which encodes the Ni-Fe hydrogenase of biotechnological interest, and that the hox operon is weakly expressed even though it possesses the two sequences resembling canonical ؊10 and ؊35 promoter boxes. In both the AbrB2-repressed promoters of the abrB2 gene and the hox operon, we found a repeated DNA motif [TT-(N 5 )-AAC], which could be involved in AbrB2 repression. Supporting this hypothesis, we found that a TT-to-GG mutation of one of these elements increased the activity of the abrB2 promoter. We think that our abrB2-deleted mutant with increased expression of the hox operon and hydrogenase activity, together with the reporter plasmids we constructed to analyze the abrB2 gene and the hox operon, will serve as useful tools to decipher the function and the regulation of hydrogen production in Synechocystis. C yanobacteria are ancient photoautotrophic prokaryotes that are regarded as the progenitors of oxygenic photosynthesis (33, 39) and the plant chloroplast (8). Over time, cyanobacteria have evolved as the largest and most diverse groups of bacteria (44) and have colonized most waters and soils of our planet. The hardiness of cyanobacteria is due to their efficient photosynthesis, which uses nature's most abundant resources, solar energy, water, CO 2 , and mineral nutrients, to produce a large part of the atmospheric oxygen and organic assimilates for the food chain (52). On a global scale, cyanobacteria fix an estimated 25 gigatons of carbon from CO 2 per year into energy-dense biomass (37, 49). To perform this huge CO 2 fixation, cyanobacteria use 0.2 to 0.3% (49) of the total solar energy, 178,000 TW, reaching the Earth's surface (22). Thus, the amount of energy passing through cyanobacteria exceeds by more than 25 times the energy demand of human society (about 15 TW), roughly 1,000 times the total nuclear energy produced on Earth.