ABSTRACT
Most expression vectors are designed to express eDNA rather than large genomic fragments because the small size of eDNA clones makes them more convenient to manipulate. These expression vectors contain principally an efficient promoter allowing a high level of transcription initiation, a polyadenylation signal or a terminator transcription sequence for limiting and terminating the transcription, and plasmid sequences to permit replication and amplification in bacterial cells. In addition to these mentioned properties, expression vectors may contain multiple specific useful elements such as: an SV40 replication origin for
amplification to high copy number in COS monkey cells, selectable markers that can be used to select cells that have stably integrated the expression vector, natural or synthetic promoter and flanking vaccinia virus DNA that determine infection and expression of vaccinia viral vectors, or yeast DNA sequences that confer autonomous replication on the expression vectors in yeast cells. The expression vectors may also contain a constitutive or an inducible promoter to allow transcription of foreign genes. In the case of inducible expression systems, several external stimuli, depending on the vectors, have been used, for instance, temperature, IPTG (isopropyl-IJ-0-thiogalactopyranoside), heavy metal ions or steroids (Ausubel et al., 1989). Although a wide variety of expression systems have been developed in the past ten years, it is difficult to compare results from the different systems. It is most important to consider the goals of the expression work.
