Journal Article
Applied and Environmental Microbiology, vol. 77, iss. 12, pp. 4035-4041, 2011
Authors
Sara M. Belchik, David W. Kennedy, Alice C. Dohnalkova, Yuanmin Wang, Papatya C. Sevinc, Hong Wu, Yuehe Lin, H. Peter Lu, James K. Fredrickson, Liang Shi
Abstract
ABSTRACT
To characterize the roles of cytochromes MtrC and OmcA of
Shewanella oneidensis
MR-1 in Cr(VI) reduction, the effects of deleting the
mtrC
and/or
omcA
gene on Cr(VI) reduction and the cellular locations of reduced Cr(III) precipitates were investigated. Compared to the rate of reduction of Cr(VI) by the wild type (wt), the deletion of
mtrC
decreased the initial rate of Cr(VI) reduction by 43.5%, while the deletion of
omcA
or both
mtrC
and
omcA
lowered the rate by 53.4% and 68.9%, respectively. In wt cells, Cr(III) precipitates were detected by transmission electron microscopy 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 scanning election microscopy on the surfaces of the wt and mutants without MtrC or OmcA but not on the mutant cells lacking both MtrC and OmcA, demonstrating that the deletion of
mtrC
and
omcA
diminishes the extracellular formation of Cr(III) precipitates. Furthermore, purified MtrC and OmcA reduced Cr(VI) with apparent
k
cat
values of 1.2 ± 0.2 (mean ± standard deviation) and 10.2 ± 1 s
−1
and
K
m
values of 34.1 ± 4.5 and 41.3 ± 7.9 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.
