As Transport Pathway
The
As transport pathway includes a total of 22 gene families with 284,186
representative sequences and 386 homologous orthology groups (Figure 3).
Among
these, the genes responsible for glycerol and phosphate transporters
(glpF , PiT ,pstA ,pstB , pstC and pstS ) can absorb As(III) and As(V)
as
their analogues into microorganisms. Gene
families
including arsA , arsB , aqpS , acr3 ,arsF , arsT , GET3 and ASNA1 participate in
As(III) efflux. As(III) efflux systems have been intensively studied in
both microbes and higher organisms (Ali, Isayenkov, Zhao, & Maathuis,
2009; Tamaki & Frankenberger, 1992; Zhu et al., 2017). In particular,
the acr3 gene family is most common in bacteria (Bobrowicz,
Wysocki, Owsianik, Goffeau, & Ułaszewski, 1997). In addition, the
gene
family arsJ encodes an organoarsenical efflux permease, in which
organic As is decomposed into As(V) and 3-phosphoglycerate when excreted
from cells. The net reaction is effectively As(V) extrusion, which is
the only known efflux pathway for As(V) (J. Chen, Yoshinaga, Garbinski,
& Rosen, 2016). Meanwhile, the gene family arsP has been
demonstrated to be an efflux system specific for trivalent
organoarsenicals
(J. Chen, Madegowda, Bhattacharjee, & Rosen, 2015). Since As(III) and
As(V) act as analogues of glycerol and phosphate, they can enter
microbial cells via glycerol transporters (GlpF ) and phosphate
transporters (Pit /Pst ), respectively.
As (V)
Respiratory Pathway
The
As respiratory pathway contains arrA and arrB gene
families with 1,498 representative sequences encoding arsenate
respiratory reductase
(Figure
3; Table S5).
The
large catalytic subunit (ArrA) and small subunit (ArrB) can form a
heterodimer (ArrAB) (Afkar et al., 2003; Krafft & Macy, 1998).
Dissimilatory As(V)-respiring prokaryotes (DARPs) have evolved pathways
to take advantage of As(V) as a terminal electron acceptor. This
energy-generating respiratory chain uses the respiratory
As(V)
reductase ArrAB, which reduces the less toxic As(V) to the more toxic
and potentially more mobile As(III)(Afkar et al., 2003; Basu, Stolz, &
Oremland,
2010).
It is
noteworthy
that As(V) respiration and As(III) oxidation functions mainly occur the
periplasm whereas As(V) reduction and As(III) methylation mainly occur
in the cytoplasm (Basu et al., 2010).