MOLECULAR CHARACTERIZATION AND OVEREXPRESSION OF THE PETF GENE FROM SYNECHOCOCCUS-ELONGATUS - EVIDENCE FOR A 2ND SITE OF ELECTROSTATIC INTERACTION BETWEEN FERREDOXIN AND THE PS I-D SUBUNIT

The petF gene from the cyanobacterium Synechococcus elongatus was isol ated using the same gene from Synechocystis sp. PCC 6803 as a heterolo gous probe. The deduced primary sequence of the isolated single copy p etF gene is identical to the primary sequence determined from the prot ein. Wild-type ferredoxin and a E93-95/Q93-95 mutant were overexpresse d in E. coli and purified. Both types of ferredoxins are photoreduced by Photosystem I and can be cross-linked to the PsaD subunit of PS I, although with reduced affinity in case of the E93-95/Q93-95 mutant. Th ese data indicate that the acidic patch of amino acids Glu94-95 of fer redoxin is most likely neither essential for the interaction of ferred oxin with PS I nor the only site of electrostatic contact with the PS I-D subunit. In contrast, NADP(+) photoreduction assays show drastical ly reduced rates in the presence of the E93-95/Q93-95 mutant ferredoxi n, indicating that these residues play a crucial role in the interacti on of ferredoxin with ferredoxin-NADP(+) reductase.