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).