## 3 Results and Discussion
### The Roles of the Genes in the Artificial Operon  
To characterize the effects of the different genes in the artificial operon on Hg accumulation, strains harboring different mer and _ppk_ genes were constructed. After the strains were cultured in medium containing Hg for a certain time span, the MRMs were measured. For Hg\({}^{2+}\) accumulation, the MRMs of BL21-B1, BL21-B2 and BL21-B1B2 were similar to that of BL21-0 (Table 4). The MRM of BL21-P was lower than those of the strains above. BL21-P accumulated more Hg than the strains described above. MerP can bind to Hg\({}^{2+}\) and plays a role as a scavenger to protect other periplasmic and membrane proteins from binding to Hg\({}^{2+}\)[21]. BL21-T transports more Hg\({}^{2+}\) than BL21-P. MerT is responsible for transporting Hg\({}^{2+}\) into cells [16]. The MRM of BL21-7 was the lowest among all the strains, indicating that BL21-7 was the most effective strain for accumulating Hg\({}^{2+}\). For MMC accumulation, the MRM of BL21-7 was much lower than that of BL21-0 (Table 4), indicating that BL21-7 could also effectively accumulate MMC. The MRMs of BL21-T, BL21-P, BL21-TP, BL21-TPK and BL21-0 were similar, demonstrating that MerT and MerP did not participate in transporting MMC. The MRM of BL21-TPK was similar to that of BL21-TP, indicating that polyphosphate could not bind to MMC. The MRMs of BL21-B1 and BL21-B2 were lower than that of \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Strains**} & \multicolumn{4}{c}{**HgCl\({}_{2}\) (umol/L)**} & \multicolumn{4}{c}{**MMC (umol/L)**} & \multicolumn{4}{c}{**PMC (umol/L)**} \\ \cline{2-13}  & **0 h** & **5 h** & **10 h** & **15 h** & **20 h** & **0 h** & **5 h** & **10 h** & **15 h** & **20 h** & **0 h** & **5 h** & **10 h** & **15 h** & **20 h** \\ \hline
**BL21-T** & 20.0 & 17.1 & 16.3 & 15.2 & 15.0 & 20.0 & 19.8 & 19.5 & 19.3 & 19.0 & 20.0 & 19.9 & 19.9 & 19.8 & 19.6 \\
**BL21-B1** & 20.0 & 19.8 & 19.8 & 19.8 & 19.4 & 20.0 & 19.8 & 19.2 & 18.5 & 18.1 & 20.0 & 18.2 & 17.5 & 16.3 & 15.5 \\
**BL21-B1** & 20.0 & 19.8 & 19.8 & 19.7 & 19.6 & 20.0 & 19.3 & 18.9 & 18.8 & 18.8 & 20.0 & 18.2 & 17.3 & 16.5 & 16.2 \\
**BL21-B1B2** & 20.0 & 19.9 & 19.8 & 19.5 & 19.2 & 20.0 & 19.5 & 18.5 & 18.0 & 17.5 & 20.0 & 17.4 & 16.5 & 15.3 & 15.1 \\
**BL21-TP** & 20.0 & 14.8 & 13.2 & 11.5 & 11.1 & 20.0 & 19.9 & 19.7 & 19.5 & 19.5 & 20.0 & 19.9 & 19.9 & 19.8 & 19.5 \\
**BL21-P** & 20.0 & 19.6 & 18.5 & 18.3 & 18.0 & 20.0 & 19.9 & 19.8 & 19.8 & 19.5 & 20.0 & 19.8 & 19.7 & 19.5 & 19.5 \\
**BL21-D** & 20.0 & 19.9 & 19.8 & 19.7 & 19.2 & 20.0 & 19.8 & 19.9 & 19.6 & 19.6 & 20.0 & 19.8 & 19.7 & 19.5 & 19.3 \\
**BL21-1** & 20.0 & 19.8 & 19.7 & 19.9 & 19.5 & 20.0 & 19.9 & 19.9 & 19.5 & 19.4 & 19.4 & 20.0 & 19.9 & 19.8 & 19.5 & 19.2 \\
**BL21-2** & 20.0 & 12.5 & 11.2 & 10.0 & 9.5 & 20.0 & 19.8 & 19.9 & 19.6 & 19.6 & 20.0 & 19.8 & 19.8 & 19.6 & 19.5 \\
**BL21-B1B2K** & 20.0 & 13.1 & 12.5 & 11.3 & 10.0 & 20.0 & 16.1 & 15.3 & 14.2 & 14.1 & 20.0 & 10.2 & 8.5 & 6.8 & 6.1 \\
**BL21-TPK** & 20.0 & 7.9 & 4.5 & 3.8 & 2.2 & 20.0 & 19.8 & 19.6 & 19.5 & 19.3 & 20.0 & 19.9 & 19.8 & 19.5 \\
**BL21-7** & 20.0 & 6.8 & 3.7 & 2.5 & 1.1 & 20.0 & 6.7 & 3.2 & 1.4 & 1.1 & 20.0 & 5.2 & 2.3 & 1.5 & 1.0 \\
**BL21-TPB1B2** & 20.0 & 14.6 & 13.2 & 12.3 & 12.2 & 20.0 & 19.5 & 19.2 & 18.3 & 17.2 & 20.0 & 17.5 & 17.1 & 15.2 & 15.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mercury remaining in the medium after strains were cultured in LB medium. Figure 9: Hg content in wastewater. CK indicates the Hg content in the wastewater sample without engineered bacteria. BL21-7 indicates the Hg content in the wastewater treated with BL21-7. BL21-0 indicates the Hg content in the wastewater treated with BL21-0. BL21-0, suggesting that MerB1 and MerB2 could also catalyze the splitting of the carbon-Hg bond in MMC. BL21-7 was the best strain for MMC removal. After BL21-7 was cultured in LB medium containing 20 umol/L MMC for 15 h, the MRM was only approximately 1.4 umol/L (Table 4).