Commonly Used Promoters

Protein synthesis, which is responsible for trait characteristics, requires genes to undergo two steps: 1) transcription, or production of a messenger RNA (mRNA); and 2) translation of mRNA into a protein (Figure 1).

Figure 1: Diagram of transcription and translation during protein synthesis

A gene has three major regions: the promoter, coding region, and terminator (Figure 2). The promoter acts as the regulator for the level of gene expression i.e. when, where and how much of the gene product (protein) is produced. The coding region contains the information for making mRNA, which in turn specifies the protein to be produced; while the terminator indicates the end of the gene.

Figure 2: Major regions of a gene

Source: http://www.phschool.com/science/biology_place/biocoach/images/transcription/tcani.gif (please visit site to see an animation of transcription)

Promoters regulate level of gene expression by specifying how many mRNAs are produced (transcribed) for a given gene. The DNA sequence of the promoter region interacts with transcription factor proteins that serve to recruit the cellular machinery needed to produce the RNA transcripts. Transcription is performed by the enzyme, RNA polymerase. The resultant RNA transcript is processed into mRNA, and then translated into protein. The number of mRNAs produced is a primary factor determining the amount of protein synthesized, which plays a role in determining the level of gene expression.

Factors that bind to promoters react to signals from the organism or/and the surrounding environment. The source and type of signal determines the type of promoters that are activated. In genetic engineering, there are three major types of promoters used, depending on the level of gene expression and specificity required:

  1. Constitutive promoters facilitate expression of the gene in all tissues regardless of the surrounding environment and development stage of the organism. Such promoters can turn on the gene in every living cell of the organism, all the time, throughout the organism’s lifetime. These promoters can often be utilized across species. Examples of constitutive promoters that are commonly used for plants include Cauliflower mosaic virus (CaMV) 35S, opine promoters, plant ubiquitin (Ubi), rice actin 1 (Act-1) and maize alcohol dehydrogenase 1 (Adh-1). CaMV 35S is the most commonly used constitutive promoter for high levels of gene expression in dicot plants. Maize Ubi and rice Act-1 are the currently the most commonly used constitutive promoters for monocots.
  2. Tissue-specific or development-stage-specific promoters facilitate expression of a gene in specific tissue(s) or at certain stages of development while leaving the rest of the organism unmodified. In the case of plants, such promoters might specifically influence expression of genes in the roots, fruits, or seeds, or during the vegetative, flowering, or seed-setting stage. If the developer wants a gene of interest to be expressed in more than one tissue type for example the root, anthers and egg sac, then multiple tissue-specific promoters may have to be included in the gene construct.
  3. Effective gene expression in specific plant parts or development stages often has been observed when promoters from closely related species are used. There are many promoters in this category because they have different tissue and developmental specificities. An example of a tissue-specific promoter is the phosphoenolpyruvate (PEP) carboxylase promoter which induces gene expression only in cells that are actively involved in photosynthesis. In plant genetic engineering, this promoter is used for traits desired in the shoot, leaves and sometimes the stem. Expression of genes controlled by this promoter is reduced later in the growing season as the plant approaches senescence.
  4. Inducible promoters are activated by exogenous (i.e., external) factors. Exogenous factors may be abiotic such as heat, water, salinity, chemical, or biotic like pathogen or insect attack. Promoters that react to abiotic factors are the most commonly used in plant genetic engineering because these can easily be manipulated. Such promoters respond to chemical compounds such as antibiotics, herbicides or changes in temperature or light. Inducible promoters can also be tissue or development stage specific.

Promoters can be derived directly from naturally occurring genes, or may be synthesized to combine regulatory sequences from different promoter regions. The promoters interact with other regulatory sequences (enhancers or silencers) and regulatory proteins (transcription factors) to influence the amount of gene transcription/expression.

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