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The unicellular cyanobacterium Synechocystis sp PCC 6803 is capable of
synthesizing two different Photosystem-I electron acceptors, ferredoxin and
flavodoxin. Under normal growth conditions a [2Fe-2S] ferredoxin was recovered
and purified to homogeneity. The complete amino-acid sequence of this protein
was established. The isoelectric point (pI = 3.48), midpoint redox potential (Em
= -0.412 V) and stability under denaturing conditions were also determined. This
ferredoxin exhibits an unusual electrophoretic behavior, resulting in a very low
apparent molecular mass between 2 and 3.5 kDa, even in the presence of high
concentrations of urea. However, a molecular mass of 10,232 Da (apo-ferredoxin)
is calculated from the sequence. Free thiol assays indicate the presence of a
disulfide bridge in this protein. A small amount of ferredoxin was also found in
another fraction during the purification procedure. The amino-acid sequence and
properties of this minor ferredoxin were similar to those of the major
ferredoxin. However, its solubility in ammonium sulfate and its reactivity with
antibodies directed against spinach ferredoxin were different. Traces of
flavodoxin were also recovered from the same fraction. The amount of flavodoxin
was dramatically increased under iron-deficient growth conditions. An acidic
isoelectric point was measured (pI = 3.76), close to that of ferredoxin. The
midpoint redox potentials of flavodoxin are Em1 = -0.433 V and Em2 = -0.238 V at
pH 7.8. Sequence comparison based on the 42 N-terminal amino acids indicates
that Synechocystis 6803 flavodoxin most likely belongs to the long-chain class,
despite an apparent molecular mass of 15 kDa determined by SDS-PAGE.
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