Salicylhydroxamic acid (SHAM) inhibits O(2) photoreduction which protects nitrogenase activity in the cyanobacterium Synechococcus sp. RF-1.

Synechococcus sp. RF-1, a unicellular N(2)-fixing cyanobacterium, can grow photosynthetically and diazotrophically in continuous light. How the organism protects its nitrogenase from damage by oxygen is unclear. In cyanobacerial cells, electron transport carriers associated with photosynthesis and respiration are all on the thylakoid membranes and share some common components, including plastoquinone pool and cytochrome b (6) f complex, and the pathways are interacting with each other. In this work, a pulse amplitude modulation (PAM) fluorometer (PAM-101) and an O(2) electrode are used simultaneously to study the chlorophyll a fluorescence and to monitor O(2) exchanges in Synechococcus sp. RF-1 cells. At the CO(2) compensation point, the photochemical quenching activity remained high unless the O(2) was exhausted by the glucose oxidase system (GOS). It indicates that in addition to CO(2), O(2) can also act as electron acceptor to receive electrons derived from Q(A). Studies with various inhibitors of the electron transport chain demonstrated that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and salicylhydroxamic acid (SHAM) inhibited the photoreduction of O(2), while glycolaldehyde, disalicylidenepropanediamine (DSPD), methyl viologen (MV) and KCN did not. These results imply that a KCN-resistant and SHAM-sensitive oxidase transfers electrons generated from Photosystem II to O(2) between cytochrome b (6) f complex and ferredoxin. When SHAM blocked this alternative electron transport pathway, the dinitrogen-fixing activity decreased significantly. The results indicate that a novel oxidase may function as an intracellular O(2)-scavenger in Synechococcus sp. RF-1 cells.