## 2 Results and Discussion
### Heavy Metal Resistance
#### 2.5.2. Copper/Arsenate/Chromium/Mercury Resistance  
Furthermore, the ZCR6 genome also carried copper resistance genes--_copA_, _copB_, _cusR_, _cusS_, and _cueR_ (Table 5). CopA and CopB are P-type Cu\({}^{+}\)-ATPases responsible for transporting monovalent Cu from the cytoplasm, which is coupled to ATP hydrolysis [72]. Such Cu transporters function in many gram-negative bacteria, e.g., _Pseudomonas aeruginosa_[73], _Vibrio cholerae_[74], and _Salmonella enterica_ sv. _typhimuriumurium_[75]. CusRS is a two-component, signal transduction system that is responsive to copper ions and activates the expression of the _cus_CFBA operon. In turn, CusCFBA is involved in periplasmic copper detoxification [76]; however, the genome of the ZCR6 strain lacks this system. In addition, genome analysis of the ZCR6 strain revealed the presence of the _cueR_ gene encoding CueR, which regulates the expression of the P-type Cu\({}^{+}\)-ATPase [74].  
Moreover, the presence of the _chrA_ gene encoding a chromate transporter that acts as an efflux pump [77] and the _chrR_ gene encoding chromate reductase that catalyzes a full reduction of Cr(VI) to Cr(III) [78] in the ZCR6 genome indicated a high degree of chromium (VI) resistance. Additionally, the arsenate resistance _arsRBC_ system (Table 5) encodes ArsR, a transcriptional regulator; ArsB, an integral membrane protein that extrudes arsenite from the cell cytoplasm; and ArsC, an arsenate reductase able to transform arsenate to arsenite prior to extrusion of this metalloid [79]. ArsC enzymes use thioredoxin or glutaredoxin as electron sources; however, in the genome of the ZCR6 strain, the presence of both  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Gene** & **Accession Number** & **KO Numbers** & **Gene Product** & **Metal Resistance** \\ \hline _arsR_ & MBG8562772.1 & K03892 & Arsenate/arsenite/antimonite-responsive & Arsenate resistance \\ _arsB_ & MBG8559938.1 & K03893 & Arsenical pump membrane protein & \\ _arsC_ & MBG8559939.1 & K03741 & Arsenate reductase (thiooredoxin) (EC:1.20.4.4) & \\ _arsC_ & MBG8562772.1 & K00537 & Arsenate reductase (glutaredoxin) (EC:1.20.4.1) & \\ _arsH_ & MBG8559940.1 & K11811 & Arsenical resistance protein arsh & \\ \hline _chrA_ & MBG8559121.1 & K07240 & Chromate transporter & Chromium resistance \\ _chrR_ & MBG8561757.1 & K19784 & Chromate reductase. Nad(p)h & \\ \hline _oprM_ & MBG8560149.1 & K18139 & Outer membrane protein. Efflux system & Multidrug resistance \\ _acrB_ & MBG8560150.1 & K18138 & Multidrug efflux pump & \\ _acrA_ & MBG8560151.1 & K03585 & Membrane fusion protein. Multidrug & \\ _acrR_ & MBG8560152.1 & K03577 & efflux system & \\ _merE_ & MBG8560549.1 & K00549 & Tetr/acrr family transcriptional regulator. Acrab & \\  & & & operon repressor & \\  & & & 5-methyltetrahydrodropretyltriglatamate- & \\  & & & homocysteine methyltransferase & Mercury resistance \\  & & & (EC:2.1.14) & \\ \hline \hline \end{tabular}
\end{table}
Table 5: _Cont._homologs has been reported. Additionally, the genome of ZCR6 carried the _arsH_ gene-encoded ArsH protein, which was demonstrated to be an organoarsenical oxidase enzyme conferring resistance to methyl As(III) derivatives in _Pseudomonas putida_ and _Sinorhizobium meliloti_[80]. It is also worth emphasizing that the genome of the ZCR6 strain carrying the _merE_ gene encodes a broad mercury transporter that mediates the transport of both Hg\({}^{2+}\) and CH\({}_{3}\)Hg(I) across bacterial membranes [81].