Eukaryotic shuttle vectors

Vectors that include replication systems derived from more than one host species are known as shuttle vectors. Such vectors commonly include a replication system able to function in E. coli and one that works in a second host, which may be bacterial or eukaryotic. Initial cloning and amplification of the DNA segment to be studied is often carried out in E. coli because it is easier to make large quantities when culturing in E. coli. The recombinant DNA molecule, consisting of the bifunctional vector plus the cloned segment of DNA, is then introduced into the second host, where the purpose is usually to measure the expression of the genes carried by the vector. Shuttle vectors that can replicate in both E. coli and yeast are the most common. 

There exist a variety of vectors for cloning into eukaryotic systems, ranging from yeast (Saccharomyces as well as Pichia) through insect cells (Baculovims) and plants (Ti plasmid from Agrobacterium tumefaciens) to mammalian cells (transfected by viral or mammalian vectors). As expression in eukaryotic hosts is less efficient than bacterial expression in terms of yield and time and more complicated in terms of vector structure and culture conditions, such eukaryotic expression systems are only used for genes whose proteins require posttranslational modification which is not possible in bacteria. Yeast is the preferred option as a relatively easily culturable single-cell system but posttranslational modification capabilities is limited. The additional complexity can be circumvented in part by exploiting the ability of eukaryotic vectors to act as shuttle vectors, which can be shuttled between two evolutionarily different hosts. Thus, eukaryotic vectors can be replicated and analyzed in bacteria and transfected into eukaryotic cells for expression of the recombinant product. 

The development of cloning vectors which propagate in eukaryotic hosts, e.g. yeast or cultured animal cells, has in particular eliminated many of the problems associates with the synthesis of eukaryotic proteins. It should be noted that post-translational processing may also vary among different eukaryontes. It is an advantage that shuttle vectors are available that are capable of propagating in both... 

The circularity of the plasmid is essential for E. coli, but not mammalian or yeast cells. This may be consistent with the circular genome of the bacteria vs linear genomes of eukaryotes. However, plasmid vectors specific for mammalian cells must be propagated, preferably in E. coli. Such shuttle vectors are therefore required to have a circular configuration. 

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