An expression system refers to the factors that work together to yield a particular gene product such as a protein, ribozyme or RNA particle.
The expression system is made up of a gene, which is encoded by DNA, and the machinery needed to make mRNA from the DNA and translate that into a protein. In this sense, every cell in the human body is an expression system. However, the term usually refers to gene expression in the laboratory.
The way a virus replicates is a good example of how an expression system may be used. The virus invades a host cell and uses the cell’s machinery as a system to generate viral gene products.
Within a gene or segment of DNA, certain sequences are referred to as promoters, enhancers and repressors. These configurations help form the expression system of the cell by regulating DNA replication and gene expression. Some of these natural expression systems have been modified for use as artificial expression systems such as the Tet-on and Tet-off expression system.
Promoters, for, example contain two distinct sequence motifs that may be around 10 bases to 35 bases upstream of the RNA transcriptional start site. The transcription initiation site is referred to as the +1 position All bases following +1 are transcribed and numbered consecutively with positive numbers. The promoter therefore lies somewhere within the -10 to -35 base region. Promoters can interact with RNA polymerase to induce the transcription process. The enzyme that synthesizes RNA (RNA polymerase) binds to promoter sequences along with transcription factors that either activate or repress promoters, therefore regulating gene expression.