Identification and characterization of PshBII, a second F_A/F_B-containing polypeptide in the photosynthetic reaction center of Heliobacterium modesticaldum

All known Type I photosynthetic reaction centers harbor three [4Fe-4S] clusters named F_X, F_A and F_B that function as terminal electron acceptors. We reported earlier that F_A and F_B in the homodimeric Type I reaction center from Heliobacterium modesticaldum reside on a loosely bound 54 amino acid protein named PshB. Time-resolved optical spectroscopy and low temperature EPR spectroscopy showed that on illumination, electrons were transferred from F_X^- to F_A and F_B at both cryogenic and room temperatures. Interestingly, the gene that codes for PshB, HM1_1462, is part of a predicted dicistronic operon that contains a second gene, named HM1_1461, which codes for a second ferredoxin-like protein with high sequence homology to PshB, including the two traditional [4Fe-4S] cluster binding motifs. RT-PCR results confirm that both genes are transcribed as a single transcript. We have cloned the HM1_1461 gene through PCR amplification of the H. modesticaldum chromosomal DNA and overexpressed the apoprotein in Escherichia coli. Reconstitution studies with inorganic reagents have shown that the holoprotein harbors ~8 iron and ~8 sulfide atoms in the form of two [4Fe-4S] clusters. Incubation of the reconstituted holoprotein with heliobacterial reaction center cores results in a charge-separated state characteristic of electron transfer past the F_X cluster to the terminal [4Fe-4S] clusters F_A and F_B. These results suggest that the HM1_1461 product, which we have named PshBII, is capable of functioning in lieu of PshB (renamed PshBI) as an alternative terminal electron transfer protein. Thus, unlike PS I, to which PsaC is tightly bound, two loosely bound ferredoxins, PshBI and PshBII, are capable of interacting with the heliobacterial reaction center. The presence of two, loosely bound F_A/F_B proteins represents a significant shift in our understanding of structure-function relationships in Type I reaction centers.