Inducer

In molecular biology, an inducer is a molecule that starts gene expression.

For a gene to be expressed, its DNA sequence must be copied (in a process known as transcription) to make a smaller, mobile molecule called messenger RNA (mRNA), which carries the instructions for making a protein to the site where the protein is manufactured (in a process known as translation). Many different types of proteins can effect the level of gene expression by promoting or preventing transcription. In prokaryotes (such as bacteria), these proteins often act on a portion of DNA known as the operator at the beginning of the gene. The operator is where RNA polymerase, the enzyme which copies the genetic sequence and synthesizes the mRNA, attaches to the DNA strand.

Inducers function by disabling repressor proteins. Repressor proteins bind to the DNA strand and prevent RNA polymerase from being able to attach to the DNA and synthesize mRNA. Inducers bind to repressors, causing them to change shape and preventing them from binding to DNA. Therefore, they allow transcription, and thus gene expression, to take place. Some inducers are modulated by activators, which have the opposite effect on gene expression as repressors. Inducers bind to activator proteins, allowing them to bind to the DNA strand where they promote RNA transcription.

Ligands that bind to deactivate activator proteins are not technically classified as inducers, since they have the effect of preventing transcription.