PNNL

Abstract

To characterize the roles of outer membrane c-type cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 in Cr(VI) reduction, the impacts of deleting mtrC and/or omcA genes on Cr(VI) reduction and cellular locations of reduced Cr(III) precipitates were investigated. Compared to wild type (wt), deletion of mtrC decreased Cr(VI) reduction by 43.1%, while deletion of omcA or both mtrC and omcA lowered the reduction by 55.8% and 57.3%, respectively. In wt cells, Cr(III) precipitates were detected by transmission electron microscopy (TEM) in the extracellular matrix between the cells, in association with the outer membrane and inside the cytoplasm. No extracellular matrix-associated Cr(III) precipitates, however, were found in the cytochrome mutant cell suspension. In mutant cells without either MtrC or OmcA, most Cr(III) precipitates were found in association with the outer membrane, while in mutant cells lacking both MtrC and OmcA, most Cr(III) precipitates were found inside the cytoplasm. Cr(III) precipitates were also detected by SEM on the surfaces of wt and mutants without MtrC or OmcA but not on the mutant cells lacking both MtrC and OmcA, demonstrating that deletion of mtrC and omcA diminishes extracellular formation of Cr(III) precipitates. Furthermore, purified MtrC and OmcA reduced Cr(VI) with apparent kcat of 1.21 ± 0.06 and 10.21 ± 1.01 s-1 and KM of 34.08 ± 4.52 and 41.27 ± 7.88 µM, respectively. Together, these results consistently demonstrate that MtrC and OmcA are the terminal reductases used by S. oneidensis MR-1 for extracellular Cr(VI) reduction where OmcA is a predominant Cr(VI) reductase.