Fig. 1. Genetic organization of a typical expression vector and improvement techniques for essential parts. A typical expression vector contains various functional genetic elements including a replication origin, promoter, selection marker, multiple cloning site (MCS), and a terminator for transcriptional termination, as well as a tag and/or fusion partner for probing and purification of the target protein. This figure briefly illustrates the functions of the three key parts (replication origin, promoter, and selection marker). The combination of these components determines the characteristics of the expression vector. Therefore, several approaches have been developed solely or combinatorially to improve the function of expression vector by replacing (cut and paste), engineering (directed evolution), and using alternatives.
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