RESPIRATORY COMPONENTS IN ACIDOPHILIC BACTERIA THAT RESPIRE ON IRON

Microorganisms capable of aerobic respiration on ferrous ions are spread throughout eubacterial and archaebacterial phyla. Phylogenetically distinct organisms were shown to express spectrally distinct redox-active biomolecules during autotrophic growth on soluble iron. A new iron-oxidizing eubacterium, designated as strain Funis, was investigated Strain Funis was judged to be different from other known iron-oxidizing bacteria on the bases of comparative lipid analyses, 16S rRNA sequence analyses, and cytochrome composition studies. When grown autotrophically on ferrous ions, Funis produced conspicuous levels of a novel acid-stable, acid-soluble yellow cytochrome with a distinctive absorbance peak at 579 nm in the reduced state. Stopped-flow spectrophotometric kinetic studies were conducted on respiratory chain components isolated from cell-free extracts of Thiobacillus ferrooxidans. Experimental results were consistent with a model where the primary oxidant of ferrous ions is a highly aggregated c-type cytochrome that then reduces the periplasmic rusticyanin. The Fe(II)-dependent, cytochrome c-catalyzed reduction of the rusticyanin possessed three kinetic properties in common with corresponding intact cells that respire on iron: the same anion specificity, a similar dependence of the rate on the concentration of ferrous ions, and similar rates at saturating concentrations of ferrous ions.