Chapter 15 – Regulation of Gene Expression
Inducible: enzymes produced only when specific chemicals are present.
Constitutive: enzymes that are produced continuously, regardless of the chemical makeup of the
Negative Control: gene expression occurs unless it is shut off by some form of a regulator molecule.
Positive Control: transcription occurs only if a regulator molecule directly stimulates RNA production.
Enzymes responsible for lactose metabolism are inducible, and lactose serves as the inducer.
Regulatory region is almost always upstream of the gene cluster it controls.
Regulatory region is known as the cis-acting site.
This site binds molecules that control transcription of a gene cluster. These molecules are
known as trans-acting molecules.
Actions at the site determine whether or not the genes are transcribed into RNA and whether
the corresponding enzymes or other protein products are synthesized from the mRNA. (can turn
on or turn off transcription).
The Lac Operon
Made up of three structural genes and a regulatory region
Genes: lacZ – encodes β-galactosidase (converts lactose to glucose and galactose)
lacY – encodes amino acid sequence of permease (enzyme that facilitates the entry of
lactose into the cell)
lacA – codes for the enzyme transacetylase Gratuitous Inducers: chemical analogs of lactose