Regulation of gene expression • References: • Stryer: “Biochemistry”, 5th Ed. • Hames & Hooper: “Instant Notes in Biochemistry”, 2nd Ed.
1. Basic principles of gene control What is gene expression? • It is a process by which a gene's information is converted into the structures and functions of a cell. For a protein encoding gene, it is a process of the transcription of a gene into mRNA, and the latter is translated into protein.
How to control gene expression? • In prokaryotic cells, gene expression is regulated primarily at the level of transcription by metabolites; • In eukaryotes it is controlled by much more complex mechanisms, including the action of protein factors, hormones, etc.
Control of gene expression in prokaryotes Structural gene R DNA R mRNA Enzyme Product Substrate
2. Regulation of Prokaryotic gene expression– the operon theory • Operons are groups of genes that function to produce proteins needed by the cell. • There are two types of genes in operons • Structural genes code for proteins needed for the normal operation of the cell. • Regulator genes code for proteins that regulate other genes.
Lactose Operon Plac : promoter; Olac: operator; lac Z:b-galactosidase; lacY: permease; Lac A: transacetylase; lac I: structural gene for lac repressor; PlacI : promoter for lac I.
Regulation of the lac operon in E. coli. The repressor binds to the operator region and prevents RNA polymerase from transcribing the structural genes Repressor mRNA Repressor
In the presence of glucose & lactose Low level of transcription
In the absence of glucose and presence of lactose active CAP: catabolite activator protein
Tryptophan Operon The trp operon encodes the genes for the synthesis of tryptophan
Regulation of the trp operon in E. coli. • Binding of Trp to the trp repressor increases the activity of the repressor, and therefore Trp is known as a co-repressor. • Expression of the trp operon is also regulated by attenuation. The attenuator region contains tandem Trp codons, which play a role to attenuate the transcription of the following genes.