Dissimilatory selenatarsenatis and S. barnesii but in later selenate

Dissimilatory reduction of selenium oxyanions
Se(VI) and Se(IV)) is significantly important in the environment and involves
conservation of metabolic energy for microorganisms (Knight et al.,2002)
because they act as terminal electron acceptors in metabolism with Se(0) and
Se(II) (Huber R, et al 2000).
The mechanism of selenate reduction varies among the microorganisms. Thaueraselenatis, Sulfurospirillum barnesii
and Bacillus arseniciselenatis
bacteria have been well-characterized and shown to respire anaerobically by
using SeO4 -2
as the terminal electron acceptor. In T.
selenatis bacterium, the reduction of selenate is exterior to the plasma
membrane by periplasmic SerABC selenite reductase with subunits A, B, C. The
electrons for the reduction of selenate are transferred from quinol oxidation
to the quinol-cytochrome c oxidoreductase (QCR) then to a cytoplasmic
cytochrome c4 and transferred to heme b in SerC subunit. Electrons from the
SerC subunit are transferred to the iron–sulfur clusters (3Fe–4S) and 3 (4Fe–4S)
in SerB and onto the 4Fe–4S cluster and Mo-complex in SerA.  SeO3-2is produced as SeO4 -2acquires
electrons from SerA subunit (Lucian
and  Barton 2017). In T.selenatis, a SeO42- , NO3–,
and NO2– respiring bacterium, complete reduction of SeO42- to Se0
occurred only when the organism was grown in the presence of both SeO4 -2
and NO3– ions. Membrane-bound nitrate reductase (Nar),
periplasmic nitrate reductase (Nap), and selenite reductase (Ser) have all been
shown to be able to catalyze the reduction of SeO4 -2
to SeO3-2 (Eswayah et al., 2016). Selenite produced during the concomitant respiration
of SeO4 -2 and NO3–, is believed to
be reduced via a periplasmic NO2– reductase (Macy 1994). Enterobacter cloacae SLD1a-1, a
facultative anaerobe isolated by Losi and Frankenberger (1997), has been
reported to operate under mechanisms very similar to that of T. selenatis. While the selenate
reductase enzyme is periplasmic in T.
selenatis, theselenate reductase is bound to the cytoplasmic membrane in B. selenatarsenatis  and S.
barnesii but in later selenate reductase has much broader substrate
specificity (Stolz and Oremland 1999). In a recent study, Li et al.,(2014) using a transposon
mutant-screening approach proposed that periplasmic fumarate reductase is a
potential enzyme involved in the respiration of selenite by Shewanella oneidensis MR-1.
Interestingly, fumaratereductase-deficient mutants did not totally suppress
selenite reduction, achieving a maximum of 60% decrease in SeO3-2
reduction compared with the wild type, suggestive of the involvement of other
enzymes or reduction strategies,  on the
other hand, neither nitrate reductase nor nitrite reductase was involved in SeO3-2
reduction to red Se0 (Muyzer and Stams 2008; Barton and Fauque
2009). It is interesting to note that while specialized selenite reductases
have been described in detail in a number of publications, a typical selenite
reductase awaits to be discovered. 

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