Does aporusticyanin mediate the adhesion of Thiobacillus ferrooxidans to pyrite?

The adhesion of Thiobacillus ferrooxidans to pyrite was quantified by electrical impedance measurements. Cells grown on soluble iron adhered specifically and with high affinity to pyrite, exhibiting an equilibrium dissociation constant of 5×10_15 M cells. Purposeful manipulation of individual cells using optical trapping techniques revealed that 92% of the iron-grown cells adhered to pyrite with a force greater than 5.2 pN, the maximum force exerted by the trap. In contrast, cells grown on sulfur adhered to pyrite with lower affinity, and 91% of sulfur-grown cells were dissociated from pyrite with an average force of 3.6 pN. Purified recombinant aporusticyanin and intact cells of T. ferrooxidans showed an identical pattern of adhesion to the same minerals. The addition of ferrous ions or organic chelators to the binding mixture prevented the binding of either aporusticyanin or intact cells to pyrite. Preincubation of either the pyrite alone or both the pyrite and the cells with exogenous aporusticyanin inhibited the adhesion of cells to pyrite by 41% and 60%, respectively. A His85Ala mutant apoprotein bound much less tightly to pyrite than did the wild type aporusticyanin. These observations are consistent with a model where aporusticyanin located on the surface of the bacterial cell acts as a mineral-specific receptor for the initial adhesion of T. ferrooxidans to pyrite. Binding of the apoprotein to solid pyrite is accomplished in part by coordination of the unoccupied copper ligands with an iron atom at the exposed edge of the pyrite crystal lattice.