| Summary: | 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 <i>bvdR</i> mutant of <i>Synechocystis</i> sp. PCC 6803 contained a secondary deletion mutation in the <i>cpcB</i> gene. The <i>bvdR</i> gene from <i>Synechococcus</i> sp. PCC 7002 was expressed in <i>Escherichia coli</i>, and recombinant BvdR was purified and shown to reduce biliverdin to bilirubin. The <i>bvdR</i> gene was successfully inactivated in <i>Synechococcus</i> sp. PCC 7002, a strain that is naturally much more tolerant of high light and ROS than <i>Synechocystis</i> sp. PCC 6803. The <i>bvdR</i> 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 <i>Synechococcus</i> 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.
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