The water-soluble
cytochrome b557 from the photosynthetic bacterium Rhodobacter capsulatus was purified and shown to have the properties of the
iron-storage
protein bacterioferritin. The molecular mass of R. capsulatus
bacterioferritin is 428 kDa and it is composed of a single type of 18-kDa subunit. The N-terminal amino acid sequence of the
bacterioferritin subunit shows 70% identity to the sequence of
bacterioferritin subunits from Escherichia coli, Nitrobacter winogradskyi, Azotobacter vinelandii and Synechocystis PCC 6803. The absorbance spectrum of reduced
bacterioferritin shows absorbance maxima at 557 nm (alpha band), 526 nm (beta band) and 417 nm (Soret band) from the six
haem groups/molecule. Antibody assays reveal that
bacterioferritin is located in the cytoplasm of R. capsulatus, and its levels stay relatively constant during batch growth under aerobic conditions when the
iron concentration in the medium is kept constant.
Iron deficiency leads to a decrease in
bacterioferritin and
iron overload leads to an increase.
Bacterioferritin from R. capsulatus has an amorphous
iron-oxide core with a high
phosphate content (900-1000 Fe atoms and approximately 600
phosphates/
bacterioferritin molecule). Mössbauer spectroscopy indicates that in both aerobically and anaerobically (phototrophically) grown cells
bacterioferritin with an Fe3+ core is formed, suggesting that
iron-core formation in vivo may not always require molecular
oxygen.