The electron donors to PSI are soluble luminal proteins. In plants, plastocyanin is the only electron donor to PSI. In addition to plastocyanin, cyanobacteria and green algae also possess a cytochrome (c553, a.k.a cyt c6)  that can donate electrons to PSI -87] - Plastocyanin is the dominant electron carrier under normal nutritional conditions. However, the copper-containing plastocyanin is replaced by cytochrome under copper- limiting conditions [88, 89] (see Chapter 8 ).
Plastocyanins show significant, yet distant, homology to other copper-containing mobile redox carriers found in FAP (auracyanin)  and Pseudomonas (azurin) with beta barrel structure -91]- The presence of plastocyanin-related proteins in distantly related organisms may be indicative of a lateral gene transfer between photosynthetic organisms . The use of plastocyanin may have allowed a separation and diversification of photosynthetic and respiratory electron transport, which share protein complexes in cyanobacteria. A diversification into different electron transport pathways is also postulated within the "redox switch hypothesis"  and could, in part, have been accomplished by different electron carriers to the RCs (see Section 13.7). There is a significant difference in the rate of electron donation from plastocyanin to PSI in cyanobacteria and plastids. The electron donation from plastid plastocyanin to PSI is twofold faster than in cyanobacteria . This increased efficiency may be due an 18 amino acid extension of the eukaryotic PsaF protein, which is part of the plastocyanin binding site. The protein extension may constitute one of the few improvements in function that have been made in plastids compared to the proto-cyanobacterial PSI.
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