Molecular Analysis of the psaC Gene Encoding the F(A)/F(B) Apoprotein of Photosystem I in the Filamentous Cyanobacterium Anabaena variabilis ATCC 29413.

In cyanobacteria, green algae, and higher plants, the psaC gene encodes PsaC, the apoprotein for two iron-sulfur clusters, FA and FB, located in the PSI complex in the thylakoid membrane. FA and FB act as the terminal electron acceptors in the PSI complex (reviewed in ref. 2). Our research objective is to develop a genetic system in which various hypotheses concerning the structural and functional roles of the PsaC protein can be tested. Our experimental organism is the filamentous nitrogen-fixing cyanobacterium Anabaena variabilis ATCC 29413 (henceforth called Anabaena 29413) for the following reasons: (a) Anabaena 29413 is capable of dark chemotrophic growth (8), and (b) it is possible to introduce exogenous DNA into these cells via conjugal transfer from Escherichia coli cells (1). In addition, positive selection vectors have been constructed so that gene replacement mutations may be created via homologous double-reciprocal recombination events (4). As a first step in the molecular genetic analysis of the PsaC protein, we recently engineered and characterized a psaC interruption mutant ofAnabaena 29413 (Table I) (5). Figure 1 shows the nucleotide sequence of the psaC gene and its flanking regions. It also shows the derived amino acid sequence of the PsaC protein. A sequence of 37 N-terminal amino acid residues of the PsaC polypeptide from a purified PSI preparation from Anabaena 29413 matches completely with the amino acid sequence deduced from the nucleotide sequence except the first Met residue (K. Nyhus, M. Ikeuchi, Y. Inoue, J. Whitmarsh, and H.B. Pakrasi, manuscript in preparation). Thus, the N-terminal Met residue ofthis protein is posttranslationally removed in Anabaena 29413. The PsaC protein from Anabaena 29413 is highly similar to the same protein from other organisms (2). The only difference in the sequence of the PsaC protein of Anabaena 29413 from that of all other species is a conservative replacement of Ile at position 38 by Val (2).