Oxygenic photosynthesis requires metal-rich cofactors and electron-transfer components that can produce reactive oxygen species (ROS) that are highly toxic to cyanobacterial cells. Biliverdin reductase (BvdR) reduces biliverdin IXα to bilirubin, which is a potent scavenger of radicals and ROS. The enzyme is widespread in mammals but is also found in many cyanobacteria. We show that a previously described bvdR mutant of Synechocystis sp. PCC 6803 contained a secondary deletion mutation in the cpcB gene. The bvdR gene from Synechococcus sp. PCC 7002 was expressed in Escherichia coli, and recombinant BvdR was purified and shown to reduce biliverdin to bilirubin. The bvdR gene was successfully inactivated in Synechococcus sp. PCC 7002, a strain that is naturally much more tolerant of high light and ROS than Synechocystis sp. PCC 6803. The bvdR mutant strain, BR2, had lower total phycobiliprotein and chlorophyll levels than wild-type cells. As determined using whole-cell fluorescence at 77 K, the photosystem I levels were also lower than those in wild-type cells. The BR2 mutant had significantly higher ROS levels compared to wild-type cells after exposure to high light for 30 min. Together, these results suggest that bilirubin plays an important role as a scavenger for ROS in Synechococcus sp. PCC 7002. The oxidation of bilirubin by ROS could convert bilirubin to biliverdin IXα, and thus BvdR might be important for regenerating bilirubin. These results further suggest that BvdR is a key component of a scavenging cycle by which cyanobacteria protect themselves from the toxic ROS byproducts generated during oxygenic photosynthesis.
Keywords: antioxidant; bilirubin; biliverdin; biliverdin reductase; cyanobacteria; photosynthesis; phycobiliproteins; reactive oxygen species; redox cycle.