Sequence of a psaC Gene from the Cyanobacterium Synechococcus sp. PCC 6301

The psaC gene encodes PsaC, the apoprotein for the terminal iron-sulfur clusters, FA and FB, in the PSI reaction center of cyanobacteria, algae, and higher plants. PsaC functions as a membrane-bound oxidoreductase, accepting electrons from the Fx iron-sulfur cluster located on the PsaA/ PsaB heterodimer and donating them to the soluble electron transfer proteins Fd and flavodoxin (reviewed in Bryant, 1992). The objective of our work is to clarify the role of PsaC in linear and cyclic electron transfer. The experimental organism is Synechococcus sp. PCC 6301, a unicellular, freshwater cyanobacterium that is used extensively for physiological, biochemical, and spectroscopic studies of photosynthesis. The PSI complex of Synechococcus sp. PCC 6301 is widely used in structural and functional studies of the FA and FB ironsulfur clusters (Parrett et al., 1990). We became interested in the psaC sequence of Synechococcus sp. PCC 6301 because there are subtle differences in the electron paramagnetic resonance spectra of the FA and FB iron-sulfur clusters between cyanobacteria and higher plants. This suggests that there may be a correlation with structure: in all organisms studied thus far, the higher-plant PsaC protein differs from the cyanobacterial PsaC protein in two regions: at position 37, higher plants contain a Lys whereas cyanobacteria contain a neutral amino acid (Gly or Ala), and at positions 70 and 71, dicots contain a Trp-His pair, monocots contain a Gly-Pro pair, and cyanobacteria contain a GlyAla pair. No sequence for the PsaC protein of Synechococcus sp. PCC 6301 is available, even though entire sequences are available for PsaD and PsaE and partial sequences are available for PsaL and PsaF. This information is relevant because