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13. Beskrive hvordan bakterier kan bruges til at udtykke eukaryote gener
Devlin, s. 309, fig. 7.22- s.310

Many plasmid and bacteriophage have been constructed to permit the expression of eukaryotic genes in bacterial cells. Rapidly replicating bacteria can serve as a biological factory to produce large amount of specific proteins.

Several important aspects have to be considered:

• The inserted foreign gene must be in the form of cDNA from its corresponding mRNA since the bacterial systems can not remove the introns on the pre-mRNA transcript. This is achieved by using reverse transcriptase. Complementary DNA molecules can be inserted into vectors that favour their efficient expression in the host cells, called expression vectors.

• The cDNA is inserted into the vector in the correct reading frame, that is insertion must occur after a triplet codon of the bacterial protein and at the beginning of the triplet codon of the eukaryotic gene protein in order to ensure proper translation

• The cDNA has to be inserted near a bacterial promoter site in order to maximise transcription. Stronger promoter – more mRNAs.

• Eukaryotic proteins synthesized within bacteria are often unstable and are quickly degraded by intracellular proteases. Fusion protein products, however, are usually stable. The fusion protein amino-acids encoded by the prokaryotic genome may be cleaved from the purified protein of interest by enzymatical or chemical procedures. An alternative can be creating a foreign protein that can be secreted, by cloning a foreign gene in a vector, such that the fusion protein synthesized contains a signal peptide that can be recognized by the bacterial signal peptidase that properly process the protein for secretion. 

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