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The promoters used for these vector are usually based on the promoter of the [[lac operon|''lac'' operon]] or the [[T7 phage|T7]] promoter,<ref>{{cite journal |vauthors=Dubendorff JW, Studier FW |title=Controlling basal expression in an inducible T7 expression system by blocking the target T7 promoter with lac repressor |journal=Journal of Molecular Biology |year=1991 |volume=219 |issue=1 |pages=45–59 |pmid=1902522 |doi=10.1016/0022-2836(91)90856-2}}</ref> and they are normally regulated by the ''lac'' [[Operator (biology)|operator]]. These promoters may also be hybrids of different promoters, for example, the [[Tac-Promoter]] is a hybrid of [[trp operon|''trp'']] and ''lac'' promoters.<ref>{{cite journal |vauthors=deBoer HA, Comstock LJ, Vasser M |year=1983|title= The tac promoter: a functional hybrid derived from trp and lac promoters |journal= Proceedings of the National Academy of Sciences USA |volume=80 |pages=21–25 |pmid=6337371 |issue=1 |pmc=393301 |doi=10.1073/pnas.80.1.21|bibcode=1983PNAS...80...21D|doi-access=free}}</ref> Note that most commonly used ''lac'' or ''lac''-derived promoters are based on the [[LacUV5|''lac''UV5]] mutant which is insensitive to [[catabolite repression]]. This mutant allows for expression of protein under the control of the ''lac'' promoter when the [[growth medium]] contains glucose since glucose would inhibit gene expression if wild-type ''lac'' promoter is used.<ref>{{cite journal |vauthors=Silverstone AE, Arditti RR, Magasanik B |title= Catabolite-insensitive revertants of lac promoter mutants |year=1970 |journal= Proceedings of the National Academy of Sciences USA |volume=66 |issue=3 |pages=773–9 |pmid=4913210 |pmc=283117 |doi=10.1073/pnas.66.3.773|bibcode= 1970PNAS...66..773S |doi-access= free }}</ref> Presence of glucose nevertheless may still be used to reduce background expression through residual inhibition in some systems.<ref>{{cite journal |url=http://wolfson.huji.ac.il/expression/procedures/bacterial/Glucose%20supression.pdf |title=Use of glucose to control basal expression in the pET System |author1=Robert Novy |author2=Barbara Morris |journal=InNovations |number=13 |pages=6–7 }}</ref>
Examples of ''E. coli'' expression vectors are the pGEX series of vectors where [[glutathione S-transferase]] is used as a fusion partner and gene expression is under the control of the tac promoter,<ref>{{cite journal |title=Single-step purification of polypeptides expressed in Escherichia coli as fusions with glutathione S-transferase |vauthors=Smith DB, Johnson KS |journal=Gene |year=1988 |volume=67|issue=1 |pages=31–40|pmid=3047011 |doi=10.1016/0378-1119(88)90005-4}}</ref><ref>{{cite web |title=GST Gene Fusion System |url=http://wolfson.huji.ac.il/purification/PDF/Tag_Protein_Purification/GST/PHARMACIA_GST_Gene_Fusion_System_Handbook.pdf |work=Amersham Pharmacia biotech }}</ref><ref>{{cite web |url=http://www.gelifesciences.com/webapp/wcs/stores/servlet/catalog/en/GELifeSciences/products/AlternativeProductStructure_16996/28954653 |title=pGEX Vectors |publisher=
It is possible to simultaneously express two or more different proteins in ''E. coli'' using different plasmids. However, when 2 or more plasmids are used, each plasmid needs to use a different antibiotic selection as well as a different origin of replication, otherwise one of the plasmids may not be stably maintained. Many commonly used plasmids are based on the [[ColE1]] replicon and are therefore incompatible with each other; in order for a ColE1-based plasmid to coexist with another in the same cell, the other would need to be of a different replicon, e.g. a p15A replicon-based plasmid such as the pACYC series of plasmids.<ref>{{cite book |title=E. coli Plasmid Vectors: Methods and Applications |author1=Nicola Casali |author2=Andrew Preston |series = Methods in Molecular Biology|volume=No.: 235 |page=22 |isbn=978-1-58829-151-6 |date=2003-07-03 }}</ref> Another approach would be to use a single two-cistron vector or design the coding sequences in tandem as a bi- or poly-cistronic construct.<ref>{{cite web |url=http://www.embl.de/pepcore/pepcore_services/cloning/cloning_methods/dicistronic_cloning/index.html |title=Cloning Methods - Di- or multi-cistronic Cloning |work=EMBL }}</ref><ref>{{cite journal |title=Translation of a synthetic two-cistron mRNA in Escherichia coli |vauthors=Schoner BE, Belagaje RM, Schoner RG |journal= Proc Natl Acad Sci U S A |year=1986 |volume=83 |issue=22|pages=8506–10 |pmid= 3534891 |pmc=386959 |doi=10.1073/pnas.83.22.8506|bibcode=1986PNAS...83.8506S |doi-access=free }}</ref>
===Yeast===
A yeast commonly used for protein production is ''[[Pichia pastoris]]''.<ref>{{cite journal |title= Recombinant protein expression in Pichia pastoris |vauthors=Cregg JM, Cereghino JL, Shi J, Higgins DR |journal=Molecular Biotechnology |year=2000 |volume=16 |issue=1 |pages=23–52 |pmid= 11098467 |doi=10.1385/MB:16:1:23|s2cid=35874864 |doi-access=free }}</ref> Examples of yeast expression vector in ''Pichia'' are the pPIC series of vectors, and these vectors use the [[AOX1]] promoter which is inducible with [[methanol]].<ref>{{cite web |url=http://tools.invitrogen.com/content/sfs/brochures/B-067202_Pichia_Flyer.pdf |title=Pichia pastoris Expression System |work=Invitrogen }}</ref> The plasmids may contain elements for insertion of foreign DNA into the yeast genome and signal sequence for the secretion of expressed protein. Proteins with disulphide bonds and glycosylation can be efficiently produced in yeast. Another yeast used for protein production is ''[[Kluyveromyces lactis]]'' and the gene is expressed, driven by a variant of the strong [[lactase]] LAC4 promoter.<ref>{{cite web |url=https://www.neb.com/~/media/Catalog/All-Products/B1A99D5EBC6E45B3B876E40A8ECCED3F/Datacards%20or%20Manuals/manualE1000.pdf |title=K. lactis Protein Expression Kit |work=New England BioLabs Inc. |access-date=2013-03-20 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304185904/https://www.neb.com/~/media/Catalog/All-Products/B1A99D5EBC6E45B3B876E40A8ECCED3F/Datacards%20or%20Manuals/manualE1000.pdf |url-status=dead }}</ref>
''[[Saccharomyces cerevisiae]]'' is particularly widely used for gene expression studies in yeast, for example in [[yeast two-hybrid system]] for the study of protein-protein interaction.<ref>{{cite journal |vauthors=Fields S, Song O |title=A novel genetic system to detect protein-protein interactions |journal=Nature |volume=340 |issue=6230 |pages=245–6 |year=1989 |pmid=2547163 |doi=10.1038/340245a0 |bibcode=1989Natur.340..245F |s2cid=4320733 }}</ref> The vectors used in yeast two-hybrid system contain fusion partners for two cloned genes that allow the transcription of a reporter gene when there is interaction between the two proteins expressed from the cloned genes.
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Many plant expression vectors are based on the [[Ti plasmid]] of ''[[Agrobacterium tumefaciens]]''.<ref>{{cite journal |title=Techniques in plant molecular biology--progress and problems |vauthors=Walden R, Schell J |journal=European Journal of Biochemistry |year= 1990 |volume=192 |issue=3 |pages=563–76 |pmid= 2209611|doi=10.1111/j.1432-1033.1990.tb19262.x |doi-access= }}</ref> In these expression vectors, DNA to be inserted into plant is cloned into the [[T-DNA Binary system|T-DNA]], a stretch of DNA flanked by a 25-bp direct repeat sequence at either end, and which can integrate into the plant genome. The T-DNA also contains the selectable marker. The ''Agrobacterium'' provides a mechanism for [[transformation (genetics)|transformation]], integration of into the plant genome, and the promoters for its ''vir'' genes may also be used for the cloned genes. Concerns over the transfer of bacterial or viral genetic material into the plant however have led to the development of vectors called intragenic vectors whereby functional equivalents of plant genome are used so that there is no transfer of genetic material from an alien species into the plant.<ref>{{cite book |url=https://books.google.com/books?id=mpc02lNJRs8C&pg=PT629 |title=Principles of Plant Genetics and Breeding|author= George Acquaah |date=16 August 2012|publisher= John Wiley & Sons Inc |isbn=978-1-118-31369-5 }}</ref>
Plant viruses may be used as vectors since the ''Agrobacterium'' method does not work for all plants. Examples of plant virus used are the [[tobacco mosaic virus]] (TMV), [[potato virus X]], and [[cowpea mosaic virus]].<ref>{{cite journal |title= Use of viral vectors for vaccine production in plants |author1=M Carmen Cañizares |author2=Liz Nicholson |author3=George P Lomonossoff |journal=Immunology and Cell Biology |year=2005 |volume=83 |issue=3 |pages= 263–270 |doi=10.1111/j.1440-1711.2005.01339.x |pmid=15877604 |pmc=7165799 }}</ref> The protein may be expressed as a fusion to the coat protein of the virus and is displayed on the surface of assembled viral particles, or as an unfused protein that accumulates within the plant. Expression in plant using plant vectors is often constitutive,<ref>{{cite web |title=How Do You Make A Transgenic Plant? |url=http://cls.casa.colostate.edu/transgeniccrops/how.html |work=Department of Soil and Crop Sciences at Colorado State University |access-date=2013-02-06 |archive-date=2013-01-21 |archive-url=https://web.archive.org/web/20130121061854/http://cls.casa.colostate.edu/TransgenicCrops/how.html |url-status=dead }}</ref> and a commonly used constitutive promoter in plant expression vectors is the [[cauliflower mosaic virus]] (CaMV) 35S promoter.<ref>{{cite journal |author1=Fütterer J. |author2=Bonneville J. M. |author3=Hohn T |title=Cauliflower mosaic virus as a gene expression vector for plants |journal=Physiologia Plantarum
|volume = 79 |issue = 1 |pages = 154–157 |date= May 1990 |doi= 10.1111/j.1399-3054.1990.tb05878.x }}</ref><ref>{{cite journal |title=The Cauliflower Mosaic Virus 35S Promoter: Combinatorial Regulation of Transcription in Plants |vauthors=Benfey PN, Chua NH |journal=Science |year=1990 |volume=250|issue=4983 |pages=959–66 |pmid=17746920 |url=http://www.sciencemag.org/site/feature/data/plants2001/PDFs/250-4983-959.pdf |doi=10.1126/science.250.4983.959|bibcode=1990Sci...250..959B |s2cid=35471862 }}</ref>
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