Expression Vectors

Once a gene has been cloned into a vector it may or may not be expressed. If both structural gene and promoter were cloned on the same segment of DNA the gene may well be expressed. On the other hand, if only the structural gene was cloned then expression will depend on whether a promoter is provided by the plasmid. Vectors that

FIGURE 22.31 Expression Vectors Can Have Tightly Regulated Promoters

An expression vector contains sequences upstream of the cloned gene that control transcription and translation of the cloned gene. The expression vector shown uses the lac promoter, which is very strong, but inducible. To stimulate transcription, an artificial inducer molecule called IPTG is added. IPTG binds to the LacI repressor protein which then detaches from the DNA. This allows RNA polymerase to bind. Before IPTG is added to the culture, the LacI repressor prevents the cloned gene from being expressed.

FIGURE 22.32 T7 RNA Polymerase System

Specialized promoters can be used to control the expression of cloned genes. In the T7 RNA poly merase system, the cloned gene cannot be expressed unless the bacterial cell makes T7 RNA polymerase. The polymerase is produced by certain genetically engineered bacteria, which have the gene encoding it inserted into the chromosome. Expression of the T7 RNA polymerase gene is under control of the lac promoter, as described in the previous figure.

^ed bacteria ih

cloned gene

T7 gene 10

promoter

(only recognized by T7 RNA polymerase)

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