Sodium selenate effect on Synechococcus elongatus PCC 7942: appearance of novel enzymatic reaction, ATP:selenate adenylyltransferase, and variation in antioxidant enzyme activities.

Synechococcus elongatus PCC 7942 was grown with SO(4) (2-) or S(2)O(3) (2-) sulfur source with varying concentration of SeO(4) (2-). Up to 150 muM, SeO(4) (2-) was growth supportive and above this it was inhibitory. Selenate is believed to induce ROS production, and to counter, the cells produce increasing amounts of ROS quenching enzymes. To investigate the differential growth response of SeO(4) (2-) at the level of ROS production, the activities of antioxidant enzymes viz. glutathione peroxidase, catalase, pyrogallol-peroxidase, superoxide dismutase and glutathione reductase were determined. Regardless of SeO(4) (2-) concentration in the medium, except catalase, the activity of other enzymes increased over SeO(4) (2-)-free controls. To understand the fate of SeO(4) (2-) added at lower concentrations, the activities of key inorganic sulfur metabolizing enzymes, ATP sulfurylase in SO(4) (2-) medium and thiosulfate reductase in S(2)O(3) (2-) medium were measured. This was done with an assumption that these enzymes would consume the SO(4) (2-) analog SeO(4) (2-) as sulfur source. We found enzymatic SeO(4) (2-)/ATP dependent formation of inorganic pyrophosphate in a reaction, analogous to ATP sulfurylase (SO(4) (2-) + ATP --> pyrophosphate) carried out by an enzyme different from ATP sulfurylase, tentatively called ATP:selenate adenylyltransferase. We hypothesize that selenium could act as trace element for S. elongatus PCC 7942.