title: The chromate-inducible chrBACF operon from the transposable element TnOtChr confers resistance to chromium(VI) and superoxide.
abstract: Large-scale industrial use of chromium(VI) has resulted in widespread contamination with carcinogenic chromium(VI). The abilities of microorganisms to survive in these environments and to detoxify chromate require the presence of specific resistance systems. Here we report identification of the transposon-located (TnOtChr) chromate resistance genes from the highly tolerant strain Ochrobactrum tritici 5bvl1 surviving chromate concentrations of >50 mM. The 7,189-bp-long TnOtChr of the mixed Tn21/Tn3 transposon subfamily contains a group of chrB, chrA, chrC, and chrF genes situated between divergently transcribed resolvase and transposase genes. The chrB and chrA genes, but not chrF or chrC, were essential for establishment of high resistance in chromium-sensitive O. tritici. The chr promoter was strongly induced by chromate or dichromate, but it was completely unresponsive to Cr(III), oxidants, sulfate, or other oxyanions. Plasmid reporter experiments identified ChrB as a chromate-sensing regulator of chr expression. Induction of the chr operon suppressed accumulation of cellular Cr through the activity of a chromate efflux pump encoded by chrA. Expression of chrB, chrC, or chrF in an Escherichia coli sodA sodB double mutant restored its aerobic growth in minimal medium and conferred resistance to superoxide-generating agents menadione and paraquat. Nitroblue tetrazolium staining on native gels showed that ChrC protein had superoxide dismutase activity. TnOtChr appears to represent a mobile genetic system for the distribution of the chromate-regulated resistance operon. The presence of three genes protecting against superoxide toxicity should provide an additional survival advantage to TnOtChr-containing cells in the environments with multiple redox-active contaminants.
authors.affiliation: Laboratorio de Microbiologia, Departmento de Bioquimica, Universidade de Coimbra, 3001-517 Coimbra, Portugal.
authors.forename: Rita
authors.initials: R
authors.lastname: Branco
authors.forename: Ana Paula
authors.initials: AP
authors.lastname: Chung
authors.forename: Tatiana
authors.initials: T
authors.lastname: Johnston
authors.forename: Volkan
authors.initials: V
authors.lastname: Gurel
authors.forename: Paula
authors.initials: P
authors.lastname: Morais
authors.forename: Anatoly
authors.initials: A
authors.lastname: Zhitkovich
mesh_terms: D001483:Base Sequence; D002840:Chromates / Q000494:pharmacology*; D002857:Chromium / Q000494:pharmacology*; D004251:DNA Transposable Elements / Q000235:genetics*; D024881:Drug Resistance, Bacterial / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects; D005798:Genes, Bacterial / Q000235:genetics; D008969:Molecular Sequence Data; D042041:Ochrobactrum / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D009876:Operon / Q000235:genetics*; D011401:Promoter Regions, Genetic / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012689:Sequence Homology, Nucleic Acid; D013481:Superoxides / Q000494:pharmacology*
chemical_list: D002840:Chromates; D004251:DNA Transposable Elements; D002857:Chromium; D013481:Superoxides
references.citation: Anal Biochem. 1971 Nov;44(1):276-87
references.pmid: 4943714
references.citation: J Bacteriol. 1992 Feb;174(3):953-61
references.pmid: 1732228
references.citation: Chem Res Toxicol. 2005 Jan;18(1):3-11
references.pmid: 15651842
references.citation: Chem Res Toxicol. 2005 Sep;18(9):1378-83
references.pmid: 16167829
references.citation: Plasmid. 1990 Nov;24(3):163-89
references.pmid: 1963947
references.citation: J Bacteriol. 1990 Jan;172(1):287-91
references.pmid: 2152903
references.citation: J Gen Microbiol. 1986 Feb;132(2):465-80
references.pmid: 3011964
references.citation: Appl Environ Microbiol. 2007 Jul;73(14):4477-83
references.pmid: 17526790
references.citation: J Bacteriol. 1983 Sep;155(3):1105-9
references.pmid: 6309741
references.citation: J Biol Chem. 1990 Apr 5;265(10):5648-53
references.pmid: 2180932
references.citation: Gene. 1995 Dec 1;166(1):175-6
references.pmid: 8529885
references.citation: Appl Environ Microbiol. 2006 Mar;72(3):2247-53
references.pmid: 16517682
references.citation: Methods Enzymol. 1994;235:386-405
references.pmid: 8057911
references.citation: J Bacteriol. 1999 Dec;181(23):7398-400
references.pmid: 10572148
references.citation: Free Radic Biol Med. 2006 Jun 1;40(11):1981-92
references.pmid: 16716899
references.citation: FEMS Microbiol Lett. 1998 Aug 15;165(2):253-60
references.pmid: 9742696
references.citation: Appl Environ Microbiol. 1998 Feb;64(2):453-8
references.pmid: 9464379
references.citation: J Biol Chem. 2004 Jul 16;279(29):30419-24
references.pmid: 15087443
references.citation: J Bacteriol. 1985 Apr;162(1):328-34
references.pmid: 3884593
references.citation: J Appl Microbiol. 2002;92(5):837-43
references.pmid: 11972686
references.citation: Can J Microbiol. 2004 Sep;50(9):697-703
references.pmid: 15644923
references.citation: FEMS Microbiol Lett. 2002 Jul 2;212(2):249-54
references.pmid: 12113942
references.citation: Biochemistry. 2002 Mar 5;41(9):3156-67
references.pmid: 11863455
references.citation: Microbiology (Reading). 2005 Sep;151(Pt 9):3027-3039
references.pmid: 16151213
references.citation: J Bacteriol. 1998 Nov;180(21):5799-802
references.pmid: 9791139
references.citation: J Bacteriol. 2006 May;188(9):3371-81
references.pmid: 16621832
references.citation: J Bacteriol. 1987 Aug;169(8):3853-6
references.pmid: 3112130
references.citation: J Bacteriol. 1977 Apr;130(1):257-65
references.pmid: 404284
references.citation: J Bacteriol. 1989 Sep;171(9):5065-70
references.pmid: 2549011
references.citation: Microbiol Mol Biol Rev. 1999 Sep;63(3):507-22
references.pmid: 10477306
references.citation: J Bacteriol. 2004 Sep;186(17):5762-74
references.pmid: 15317781
references.citation: Arch Microbiol. 2002 Dec;179(1):15-25
references.pmid: 12471500
references.citation: FEMS Microbiol Rev. 2001 May;25(3):335-47
references.pmid: 11348688
references.citation: Nucleic Acids Res. 1997 Sep 1;25(17):3389-402
references.pmid: 9254694
references.citation: Biochemistry. 2003 Feb 4;42(4):1062-70
references.pmid: 12549927
references.citation: Mol Genet Genomics. 2003 Feb;268(5):570-84
references.pmid: 12589432
references.citation: J Biol Chem. 1999 Feb 12;274(7):4202-6
references.pmid: 9933617