Directed mutagenesis of an iron-sulfur protein of the photosystem I complex in the filamentous cyanobacterium Anabaena variabilis ATCC 29413

In oxygenic photosynthetic organisms the PSI-C polypeptide, encoded by the psaC gene, provides the ligands for two [4Fe-4S] centers, F_A and F_B, the terminal electron acceptors in the photosystem I (PSI) complex. An insertion mutation introduced in thepsaC locus of the filamentous cyanobacterium Anabaena variabilis ATCC 29413 resulted in the creation of a mutant strain, T398-1, that lacks the PSI-C polypeptide. In medium supplemented with 5 mM fructose, the mutant cells grew well in the dark. However, when grown in the same medium under light, the doubling rate of T398-1 cells was significantly decreased. In intact cells of T398-1, bicarbonate-dependent whole-chain electron transport (PSII and PSI) could not be detected, although partial electron transport reactions involving either one of the two photosystems could be measured at significant rates. The low-temperature EPR signals attributed to the [4Fe-4S] centers F_A and F_B were absent in the mutant cells. Chemical titration measurements indicated that the ratios of chlorophyll to the primary donor P700 were virtually identical in membranes from the wild-type and mutant cells. Moreover, room-temperature optical spectroscopic analysis of the thylakoid membranes isolated from T398-1 showed flash-induced P700 oxidation followed by dark rereduction, indicating primary photochemistry in PSI. Thus stable assembly of the reaction center of PSI can occur in the absence of the Fe-S cluster cofactors F_A and F_B. These studies demonstrate that Anabaena 29413 offers a useful genetic system for targeted mutagenesis of the PSI complex. (.