## 1 Introduction  
Mercury (Hg) pollution threatens public health. In nature, Hg exists in three forms: elemental (Hg(0)), inorganic (Hg\({}^{2+}\) or Hg\({}^{+}\)) and organic forms (R-Hg\({}^{+}\) or R-Hg-X, where \"R\" is methyl or phenyl and \"X\" is acetate) [1]. Elemental Hg is volatile. Inorganic Hg commonly exists as mercuric sulfide and is used as a coloring agent in tattoos and paints. All the forms are hazardous but the extent of toxicity varies among different forms of Hg. Organic Hg is the most toxic and elemental is the least toxic. Elemental Hg can potentially become an inhalation hazard. Organic Hg can impair neurological development in fetuses and young children and can damage the central nervous system of adults [2,3]. Exposure to inorganic Hg may damage the gastrointestinal tract, the nervous system and the kidneys [4]. The Hg in agricultural soil can enter the human body through crops and Hg mainly comes from irrigated-polluted water released from factories [5]. The best resolution for this problem is removing Hg from polluted water before it is discharged from factories.  
Engineered bacterial strains can be used to remove Hg from polluted water. After _E. coli_ JM109 cells, transformed with the mouse metallothione gene _mt-I_ were cultured in LB medium containing 120 \(\upmu\)mol/L mercuric chloride (HgCl\({}_{2}\)) for 72 and 120 h, the bacteria accumulated 51.6 \(\upmu\)mol/L Hg and 1002 \(\upmu\)mol/L Hg, respectively [6]. However, wild-type untransformed _E. coli_ JM109 cells(the control) could not grow at all in LB medium containing 120 umol/L HgCl\({}_{2}\)[6]. However, it is unclear whether the strain accumulated organic Hg, a main pollution component in wastewater [6]. Bacteria harboring pMRA17 containing the _Pseudomonas_ K-62 genes _merR_, _merT_, _merA_ and _merB1_ had a higher methylmercury tolerance than the control [7]. However, their Hg accumulation was achieved by volatilizing Hg(0) into the air, which causes public anxiety. The same problem exists in the _Cupiravidus metallidurans_ strain MSR33, which harbors two large plasmids (pMOL28 and pMOL30) carrying genetic determinants for heavy metal resistance [8]. Although bacteria harboring pMRB01, which contains _Pseudomonas_ K-62 genes _merR_, _merB2_ and _merD_, had a higher tolerance to HgCl\({}_{2}\) than the control, they were hypersensitive to phenylmercury [7]. _E. coli_ harboring pMKB18, which contains _Pseudomonas_ K-62 genes _merR_, _merT_, _merP_ and _merB1_ as well as the _Klebsiella aerogenes_ polyphosphate kinase gene _ppk_ could remove 55% of the Hg\({}^{2+}\) and 31% of the C\({}_{6}\)H\({}_{5}\)Hg\({}^{+}\) when it was cultured in medium containing 80 umol/L Hg\({}^{2+}\) or 80 umol/L C\({}_{6}\)H\({}_{5}\)Hg\({}^{+}\) for 20 h [9] but whether it can accumulate CH\({}_{3}\)Hg\({}^{+}\) has not been reported. CM111 and HG3, two bacteria isolated from Hg-polluted rivers or sand, also had high minimum inhibitory concentration (MIC) values of Hg\({}^{2+}\)[10]. Although the tolerance of CM111 and HG3 to Hg\({}^{2+}\) was not due to Hg(0) volatilization [10], the detailed mechanisms of these two strains' tolerance to Hg\({}^{2+}\) are unclear. Whether these strains can remove organic Hg from a medium is also unknown.  
The aim of this study was to construct an engineered bacterial strain for accumulating Hg from liquid medium. This bacterial strain should meet the following criteria: (1) it can accumulate both mercuric ions and organic mercury; (2) it has a clear genetic background; (3) its mechanisms for mercury accumulation are definite; and (4) it can keep mercury accumulated in cells. To achieve this aim, an artificial operon, _P165-g10-merT-merP-merB1-merB2-ppk-rpsT_ and a bacterial strain, BL21-7, were constructed. The role of the genes in the artificial operon and the mechanism by which the bacteria accumulated mercury were also studied.