Regulation of gene expression References: 1.Stryer: “Biochemistry”, 5 th Ed. 2.Hames & Hooper: “Instant Notes in Biochemistry”, 2 nd 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  R Structural gene Substrate Product Enzyme mRNA DNA R

Control of gene expression in eukaryotes

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 A.Structural genes code for proteins needed for the normal operation of the cell. B.Regulator genes code for proteins that regulate other genes.

P lac : promoter; O lac : operator; lac Z:  -galactosidase; lacY: permease; Lac A: transacetylase; lac I: structural gene for lac repressor; P lacI : promoter for lac I. Lactose Operon

Regulation of the lac operon in E. coli. Repressor mRNA Repressor The repressor binds to the operator region and prevents RNA polymerase from transcribing the structural genes

In the presence of lactose mRNA

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.