The bound electron acceptors in green sulfur bacteria:: Resolution of the g-tensor for the FX iron-sulfur cluster in Chlorobium tepidum

The photosynthetic reaction center (RC) of green sulfur bacteria contains two [4Fe-4S] clusters named F-A and F-B, by analogy with photosystem I (PS I). PS I also contains an interpolypeptide [4Fe-4S] cluster named F-X; however, spectroscopic evidence for an analogous iron-sulfur cluster in green sulfur bacteria remains equivocal. To minimize oxidative damage to the iron-sulfur clusters, we studied the sensitivity of F-A and F-B to molecular oxygen in whole cells of Chlorobium vibrioforme and Chlorobium tepidum and obtained highly photoactive membranes and RCs from Cb. tepidum by adjusting isolation conditions to maximize the amplitude of the F-A(-)/F-B(-) electron paramagnetic resonance signal at g = 1.89 (measured at 126 mW of microwave power and 14 K) relative to the P840(+) signal at g = 2.0028 (measured at 800 mu W of microwave power and 14 K). In these optimized preparations we were able to differentiate F-X(-) from F-A(-)/F-B(-) by their different relaxation properties. At temperatures between 4 and 9 K, isolated membranes and RCs of Cb. tepidum show a broad peak at g = 2.12 and a prominent high-field trough at g = 1.76 (measured at 126 mW of microwave power). The complete g-tensor of F-X(-), extracted by numerical simulation, yields principal values of 2.17, 1.92, and 1.77 and is similar to F-X in PS I. An important difference from PS I is that because the bound cytochrome is available as a fast electron donor in Chlorobium, it is not necessary to prereduce F-A and F-B to photoaccumulate F-X(-).