The genome of H. zincidurans strain B6T contains 31 genes related to heavy metal resistance, especially to zinc resistance
(Table 6). Zinc is an essential but also toxic metal for living being 2,43]. The concentration of zinc inside bacterial cells is maintained by importing limitation,
efflux, accumulation and sequestration 44,45]. H. zincidurans strain B6T possesses four heavy metal translocating P-type ATPases (HALZIN_733, HALZIN_1240,
HALZIN_2196 and HALZIN_2262), which may participate in the transport of Zn2+, Mn2+, Cu2+, Cd2+, Pb2+, Ag?+?and Hg2+ against the concentration gradient to the periplasm 2,44]. Especially the two ZntA P-type ATPases (HALZIN_733 and HALZIN_2196) may mediate
resistance to Zn2+, Cd2+ and Pb2+46,47]. Zn2+, Co2+, Cu2+, Cd2+ and Ni2+ are able to be transported by RND family efflux transporter protein (HALZIN_54, HALZIN_1411,
HALZIN_2047, HALZIN_2208 and HALZIN_2209) from both the cytoplasm and the periplasm
to outside 2,44]. Usually the P-type ATPases are regulated by MerR family regulators responding to
the intracellular heavy metal concentration 44,48,49]. Six analogues of MerR family regulators (HALZIN_399, HALZIN_922, HALZIN_2261, HALZIN_2264,
HALZIN_2469 and HALZIN_2675) were found in the genome of H. zincidurans strain B6T. Additionally, a zinc uptake regulation protein ZUR (HALZIN_1413), which is a repressor
regulator during zinc uptake, is also detected 44,50]. The presence of these genes is accordance with zinc resistance phenotype of H. zincidurans strain B6T.
Table 6. Description of the genes related to heavy metal resistance
Among the 31 ORFs related to heavy metal resistance, it is noteworthy of two mer-operons. One mer-operon encodes a mercuric transport protein (MerE, HALZIN_916) for organic mercury
uptake 51], a transcriptional regulator (MerD, HALZIN_917), three alkylmercury lyases (MerB,
HALZIN_918-920) catalyzing organomercurials yielding Hg2+52] and a transcriptional regulator (MerR, HALZIN_922). The other one encodes a transcriptional
regulator (MerR, HALZIN_2469), two mercuric transport proteins (MerT and MerP, HALZIN_2470-2471)
for inorganic mercury uptake 51] and a mercuric reductase (MerA, HALZIN_2472) catalyzing Hg2+ to Hg053]. According to the genomic data, H. zincidurans strain B6T is able to survive in both inorganic and organic mercury environments. Interestingly,
the four ORFs of the inorganic mer-operon showed the highest sequence identities to those of Halomonas lutea. Nevertheless, all the six ORFs of the organic mer-operon did not show the highest sequence identities to those of the genus Halomonas, but to the genera Burkholderia, Pseudomonas, Gladiecola and Stenotrophomonas, which indicates that the organic mer-operon might be acquired by HGT. Of special interest are the three alkylmercury lyases
(MerB, HALZIN_918-920), which had obvious differences between the G+C content (56.6%;
57.1, 56.6 and 56.0% for these three gene sequences, respectively) as well as the
G+C content at the third-codon positions (60.3%; 60.4, 61.0 and 59.4% for these three
gene sequences, respectively) and those of the total protein-coding genes (65.4 and
82.8%, respectively). Besides, the RSCUs of nearly half of the 59 codons used by the
three genes (23, 27 and 26 codons for HALZIN_918-920, respectively) change more than
2 folds, compared with those used by total protein-coding genes. 13 of the 31 ORFs
(41.9%) were not related to Halomonadaceae genes according to the gene sequence similarity analysis, 9 of the 13 ORFs had RSCU
change larger than 2 folds in more than 25% codons. These results indicated the existence
of HGT events among the heavy metal resistance-related genes. Thus, HGT events might
be an important way for H. zincidurans strain B6T to acquire heavy metal resistant ability and to adapt to the heavy metal rich environment.