| Summary: | Ferritins are iron storage proteins assembled from 24 subunits into a spherical and hollow structure. The genomes of many bacteria harbor genes encoding two types of ferritin-like proteins, the bacterial ferritins (Ftn) and the bacterioferritins (Bfr), which bind heme. The genome of <i>P. aeruginosa</i> PAO1 (like the genomes of many bacteria) contains genes coding for two different types of ferritin-like molecules, <i>ftnA</i> (PA4235) and <i>bfrB</i> (PA3531). The reasons for requiring the presence of two distinct types of iron storage protein in bacterial cells have remained largely unexplained. Attempts to understand this issue in <i>P. aeruginosa</i> through the recombinant expression of the <i>ftnA</i> and <i>bfrB</i> genes in <i>E. coli</i> host cells, coupled to the biochemical and structural characterization of the recombinant 24-mer FtnA and 24-mer BfrB molecules, have shown that each of the recombinant molecules can form an Fe<sup>3+</sup>-mineral core. These observations led to the suggestion that 24-mer FtnA and 24-mer BfrB molecules coexist in <i>P. aeruginosa</i> cells where they share iron storage responsibilities. Herein, we demonstrate that <i>P. aeruginosa</i> utilizes a single heterooligomeric 24-mer Bfr assembled from FtnA and BfrB subunits. The relative content of the FtnA and BfrB subunits in Bfr depends on the O<sub>2</sub> availability during cell culture, such that Bfr isolated from aerobically cultured <i>P. aeruginosa</i> is assembled from a majority of BfrB subunits. In contrast, when the cells are cultured in O<sub>2</sub>-limiting conditions, the proportion of FtnA subunits in the isolated Bfr increases significantly and can become the most abundant subunit. Despite the variability in the subunit composition of Bfr, the 24-mer assembly is consistently arranged from FtnA subunit dimers devoid of heme and BfrB subunit dimers each containing a heme molecule.
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