GENE REGULATION GENE REGULATION Virtually every cell in

GENE REGULATION

GENE REGULATION Virtually every cell in your body contains a complete set of genes n But they are not all turned on in every tissue n Each cell in your body expresses only a small subset of genes at any time n During development different cells express different sets of genes in a precisely regulated fashion n

GENE REGULATION n Gene regulation occurs at the level of transcription or production of m. RNA n A given cell transcribes only a specific set of genes and not others n Insulin is made by pancreatic cells

CENTRAL DOGMA n Genetic information always goes from DNA to RNA to protein n Gene regulation has been well studied in E. coli n When a bacterial cell encounters a potential food source it will manufacture the enzymes necessary to metabolize that food

Gene Regulation n In addition to sugars like glucose and lactose E. coli cells also require amino acids n One essential aa is tryptophan. n When E. coli is swimming in tryptophan (milk & poultry) it will absorb the amino acids from the media n When tryptophan is not present in the media then the cell must manufacture its’ own amino acids

Trp Operon n E. coli uses several proteins encoded by a cluster of 5 genes to manufacture the amino acid tryptophan n All 5 genes are transcribed together as a unit called an operon, which produces a single long piece of m. RNA for all the genes Regulatory Gene R Inactive repressor (apo-repressor) Operon P O L E D C 5 Proteins B A

RNA polymerase binds to a promoter located at the beginning of the first gene and proceeds down the DNA transcribing the genes in sequence n The tryptophane gene is turned on when there is no tryptophan in the media n the trp gene is a repressible gene (Genes whose expression is turned off by the presence of some substance (co-repressor)) n That is when the cell wants to make its’ own tryptophan n

Fig. 16. 6

Tryptophan Operon Absence of Tryptophan n Co-repressor -tryptophan n Gene expression Presence of tryptophan n P O L E D C B A Absence of tryptophan n n R Inactive repressor (apo-repressor) Activates repressor No gene expression 5 Proteins Presence of Tryptophan Negative control R P O L E D C No trp m. RNA Inactive repressor (apo-repressor) Trp (co-repressor)

GENE REGULATION n In addition to amino acids, E. coli cells also metabolize sugars in their environment n In 1959 Jacques Monod and Fracois Jacob looked at the ability of E. coli cells to digest the sugar lactose

GENE REGULATION n In the presence of the sugar lactose, E. coli makes an enzyme called beta galactosidase n Beta galactosidase breaks down the sugar lactose so the E. coli can digest it for food n It is the LAC Z gene in E coli that codes for the enzyme beta galactosidase

Lac Z Gene Lac Z is an inducible gene ( Genes whose expression is turned on by the presence of some substance) n E. coli cells can not make the sugar lactose n They can only have lactose when it is present in their environment n Then they turn on genes to break down lactose n

GENE REGULATION The E. coli bacteria only needs beta galactosidase if there is lactose in the environment to digest n There is no point in making the enzyme if there is no lactose sugar to break down n It is the combination of the promoter and the DNA that regulate when a gene will be transcribed n

GENE REGULATION n This combination of a promoter and a gene is called an OPERON n Operon is a cluster of genes encoding related enzymes that are regulated together Regulatory Gene i Operon p o z y a DNA m-RNA Protein Transacetylase b-Galactosidase Permease

LAC Z GENE E. coli regulate the production of Beta Galactocidase by using a regulatory protein called a repressor n The repressor binds to the lac Z gene at a site between the promotor and the start of the coding sequence n The site the repressor binds to is called the operator n

LAC Z GENE Normally the repressor sits on the operator repressing transcription of the lac Z gene n In the presence of lactose the repressor binds to the sugar and this allows the polymerase to move Absence of lactose n o p i y z a Active No lac m. RNA Presence of lactose i p o z y a Inactive b-Galactosidase Permease Transacetylase

LAC Z GENE n When there is no sugar left the repressor will return to its spot on the chromosome and stop the transcription of the lac Z gene Absence of lactose i p o z y Active No lac m. RNA a

Negative control (bound repressor inhibits transcription)

Catabolite Repression (Glucose Effect) Units of galactosidase Definition: Control of an operon by glucose n Catabolic operons n - glucose Glucose added + glucose Time (hr) + lactose

Mechanism of Catabolite Repression Absence of glucose c-AMP n CAP (CRP) protein n CAP-c. AMP complex n n Promoter activation Adenyl cyclase c-AMP ATP CAP i p z o y a Active Inactive b-Galactosidase Permease Transacetylase Maximum expression Positive control (bound activator facilitates trancription

Mechanism of Catabolite Repression Glucose : c. AMP n CAP (CRP) protein n No CAP-c. AMP complex Presence of glucose n n No Promoter activation Adenyl cyclase ATP X CAP i p o z y a Inactive b-Galactosidase Permease Transacetylase Low level expression

GENE REGULATION In eukaryotic organisms like ourselves there are several methods of regulating protein production n Most regulatory sequences are found upstream from the promoter n Genes are controlled by regulatory elements in the promoter region that act like on/off switches or dimmer switches n

GENE REGULATION Specific transcription factors bind to these regulatory elements and regulate transcription n Regulatory elements may be tissue specific and will activate their gene only in one kind of tissue n Sometimes the expression of a gene requires the function of two or more different regulatory elements n