Types of promoters used to regulate gene expression
Promoters used in biotechnology are of different types according to the intended type of control of gene expression. They can be generally divided into:
- Constitutive promoters. These promoters direct expression in virtually all tissues and are largely, if not entirely, independent of environmental and developmental factors. As their expression is normally not conditioned by endogenous factors, constitutive promoters are usually active across species and even across kingdoms.
- Tissue-specific or development-stage-specific promoters. These direct the expression of a gene in specific tissue(s) or at certain stages of development. For plants, promoter elements that are expressed or affect the expression of genes in the vascular system, photosynthetic tissues, tubers, roots and other vegetative organs, or seeds and other reproductive organs can be found in heterologous systems (e.g. distantly related species or even other kingdoms) but the most specificity is generally achieved with homologous promoters (i.e. from the same species, genus or family). This is probably because the coordinate expression of transcription factors is necessary for regulation of the promoter’s activity.
- Inducible promoters. Their performance is not conditioned to endogenous factors but to environmental conditions and external stimuli that can be artificially controlled. Within this group, there are promoters modulated by abiotic factors such as light, oxygen levels, heat, cold and wounding. Since some of these factors are difficult to control outside an experimental setting, promoters that respond to chemical compounds, not found naturally in the organism of interest, are of particular interest. Along those lines, promoters that respond to antibiotics, copper, alcohol, steroids, and herbicides, among other compounds, have been adapted and refined to allow the induction of gene activity at will and independently of other biotic or abiotic factors.
- Synthetic promoters. Promoters made by bringing together the primary elements of a promoter region from diverse origins.
Apart from the promoter types mentioned above, there are regulatory expression systems based on transactivating proteins. These systems regulate the expression of genes of interest irrespective of their physical position to the target genes. In fact, several chemical-inducible promoters incorporate transactivating proteins and constitutive promoters as part of the regulatory system. Transactivating proteins constitute a whole realm of molecules in the field of gene regulation and deserve a separate analysis. We deal with some of them in this paper, but only to the extent that they overlap with the types of promoters aforementioned.