Editing Expression vector (section)

===Elements for expression===
{{further|Transcription (genetics)|Translation (biology)}}
An expression vector must have elements necessary for gene expression. These may include a [[Promoter (genetics)|promoter]], the correct translation initiation sequence such as a [[ribosomal binding site]] and [[start codon]], a [[termination codon]], and a [[Terminator (genetics)|transcription termination sequence]].<ref>{{cite book |title=Principles of Gene Manipulation |chapter-url=https://archive.org/details/principlesofgene00oldr |chapter-url-access=registration |chapter=Chapter 8: Expression E. coli of cloned DNA molecules |author=RW Old |author2=SB Primrose |year=1994 |publisher=Blackwell Scientific Publications |isbn=978-0-632-03712-4 }}</ref> There are differences in the machinery for protein synthesis between prokaryotes and eukaryotes, therefore the expression vectors must have the elements for expression that are appropriate for the chosen host.  For example, prokaryotes expression vectors would have a [[Shine-Dalgarno sequence]] at its translation initiation site for the binding of ribosomes, while eukaryotes expression vectors would contain the [[Kozak consensus sequence]].

The [[Promoter (genetics)|promoter]] initiates the [[Transcription (genetics)|transcription]] and is therefore the point of control for the expression of the cloned gene. The promoters used in expression vector are normally [[Enzyme induction and inhibition|inducible]], meaning that protein synthesis is only initiated when required by the introduction of an [[inducer]] such as [[IPTG]]. Gene expression however may also be constitutive (i.e. protein is constantly expressed) in some expression vectors. Low level of constitutive protein synthesis may occur even in expression vectors with tightly controlled promoters.