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The '''''nik operon''''' is an [[operon]] required for uptake of nickel ions into the cell. It is present in many bacteria, but has been extensively studied in ''[[Helicobacter pylori]]''. [[Nickel]] is an essential nutrient for many microorganisms, where it participates in a variety of cellular processes. However, excessive levels of nickel ions in cell can be fatal to the cell. Nickel ion concentration in the cell is regulated through the ''nik'' operon.

== Structure of the ''nik'' operon ==
The ''nik'' operon consists of six genes. The first five genes ''nikABCDE'' encode components of a typical [[ATP-binding cassette transporter|ABC transport system]] and the last gene ''nikR'' encodes a DNA-binding protein that represses transcription of ''nikABCDE'' when sufficient Ni2+ is present. The ''nikR'' gene is located 5 bp downstream of the end of ''nikE'', transcribed in the same direction as ''nikABCDE''. The following table summarizes the structure of the ''nik'' operon:
{| class="wikitable sortable"
|-
! Gene !! Protein encoded !! Functional Role
|-
| ''nikA'' || ABC transporter Ni2+binding component || binds Ni2+ in the [[periplasm]] and transfers it to the [[permease]]; may also play a role in [[chemotaxis]] away from toxic concentrations of nickel
|-
| ''nikB'' & ''nikC'' || ABC transporter permease components || transport of Ni2++ across the inner membrane
|-
| ''nikD'' & ''nikE'' || ABC transporter ATP-binding components || [[ATPase|hydrolyzes ATP]] to provide energy for the transport process
|-
| ''nikR'' || transcriptional regulator of the ''nikABCDE'' operon || downregulates expression of the ''nik'' operon in the presence of excess Ni2+ ions
|}

== Regulation ==
=== ''nikR'' Regulation ===
Regulation of expression of the ''nikR'' gene is achieved by two [[promoter (genetics)|promoter]]s. The first is through the [[FNR regulon]]. The FNR controlled regulation of ''nikABCDE–nikR'' occurs at a FNR box located upstream of ''nikA'' at a putative NikR binding site.<ref name=a>{{cite journal|last=Navarro|first=Clarisse|author2=Wu, Long-Fei |author3=Mandrand-Berthelot, Marie-Andrée |title=The ''nik'' operon of ''Escherichia coli'' encodes a periplasmic binding-protein-dependent transport system for nickel|journal=Molecular Microbiology|date=1 September 1993|volume=9|issue=6|pages=1181–1191|doi=10.1111/j.1365-2958.1993.tb01247.x|pmid=7934931 }}</ref> The second promoter element regulating ''nikR'' expression occurs 51 bp upstream of the ''nikR'' transcription start site and results in low-level constitutive expression. There is also evidence that ''nikR'' expression is partially autoregulated.<ref name=b>{{cite journal|last=De Pina|first=K|author2=Desjardin, V |author3=Mandrand-Berthelot, MA |author4=Giordano, G |author5= Wu, LF |title=Isolation and characterization of the nikR gene encoding a nickel-responsive regulator in Escherichia coli.|journal=Journal of Bacteriology|date=January 1999|volume=181|issue=2|pages=670–4|doi=10.1128/JB.181.2.670-674.1999|pmid=9882686 |pmc=93426}}</ref>

=== Regulation of Ni2+ uptake ===
Ni2+ is taken up into prokaryotic cells by one of two types of high-affinity transport systems.<ref name=c>{{cite journal|last=Eitinger|first=T|author2=Mandrand-Berthelot, MA |title=Nickel transport systems in microorganisms.|journal=Archives of Microbiology|date=January 2000|volume=173|issue=1|pages=1–9|pmid=10648098|doi=10.1007/s002030050001|bibcode=2000ArMic.173....1E}}</ref> The first method involves ABC-type transporters (discussed in this article) and the second mechanism makes use of transition-metal permeases (such as HoxN of ''[[Ralstonia eutropha]]''). The ABC-type transporter system consists of five proteins, NikA–E, that carry out the ATP-dependent transport of Ni2+.<ref name=a/> NikA is a soluble, periplasmic, Ni-binding protein; NikB and NikC form a transmembrane pore for passage of Ni; and NikD and NikE hydrolyze ATP and couple this energy to Ni2+-transport. When Ni2+ is available in excess, NikR protein represses transcription of ''nikABCDE''.<ref name=b />

[[File:Regulation of nickel through nik operon.png|thumb|center|500px|Regulation of nickel ion uptake by components of the ''nik'' operon]]

== Repression by NikR binding ==
Using profile-based sequence database searches, NikR was shown to be a member of the ribbon-helix-helix (RHH) family of transcription factors.<ref>{{cite journal|last=Chivers|first=PT|author2=Sauer, RT |title=Regulation of high affinity nickel uptake in bacteria. Ni2+-Dependent interaction of NikR with wild-type and mutant operator sites.|journal=The Journal of Biological Chemistry|date=Jun 30, 2000|volume=275|issue=26|pages=19735–41|pmid=10787413|doi=10.1074/jbc.m002232200|doi-access=free}}</ref> It has been demonstrated that the N-terminal domain of NikR is responsible for binding to DNA and that it only binds in presence of Ni2+. NikR has two sites for binding to Ni2+ ions. Binding of Ni2+ at concentrations that allow full occupancy of only the high-affinity sites is sufficient for operator binding, but the affinity for the operator is increased 1000-fold and the operator footprints are larger when both nickel-binding sites are occupied.<ref>{{cite journal|last=Chivers|first=Peter T.|author2=Sauer, Robert T. |title=NikR Repressor|journal=Chemistry & Biology|date=1 October 2002|volume=9|issue=10|pages=1141–1148|doi=10.1016/s1074-5521(02)00241-7|doi-access=free|pmid=12401498 }}</ref> These results, combined with estimates of intracellular Ni2+ and NikR concentrations, lead to the conclusion that NikR is able to sense Ni2+ and regulate the ''nik'' operon expression over a range of intracellular Ni2+ concentrations from as low as one to as high as 10,000 molecules per cell.

== See also ==
* [[gene regulation]]
* [[Operon]]
* [[lac operon]]

== References ==
{{Reflist}}

[[Category:Genetics]]
[[Category:Nickel]]
[[Category:Operons]]

Nik operon - История изменений

Admin: 1 версия импортирована

ru>Citation bot: Added bibcode. | Use this bot. Report bugs. | Suggested by Abductive | Category:Operons | #UCB_Category 10/13