Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Control of Gene Expression Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Outline Overview of Transcriptional Control Reading a Helix DNA Binding Motifs Controlling Transcription Initiation Designing Complex Gene Control System Chromosome Structure and Gene Regulation Posttranscriptional Control in Eukaryotes

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Overview of Transcriptional Control Regulating Promoter Access  Binding protein to regulatory sequence blocks transcription by getting in the way of RNA polymerase, or stimulates transcription by facilitating binding of RNA polymerase to the promoter. Transcriptional Control in Prokaryotes  Adjust cell’s activities to immediate environment.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Overview of Transcriptional Control Transcriptional Control in Eukaryotes  Primary function of gene control is maintaining homeostasis. - Compensate for physiological changes, or mediate production decisions. Posttranscriptional Control  Control of transcription by RNA polymerase  Influencing mRNA produced from genes or the activity of proteins encoded by the mRNA.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Reading a Helix Helix can be read w/out unwinding. Major Groove  Proteins can bind to outside surface of DNA helix. - Nucleotide’s hydrophobic methyl, hydrogen atoms, and hydrogen bond donors and acceptors protrude from major groove. - Employ particular bends in protein chain that permit interlocking with major groove (DNA-Binding Motifs).

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Four Important DNA-Binding Motifs: Helix-Turn-Helix Motif  One helical segment fits snugly in major groove, and the other butts against the outside of the DNA molecule. - Two binding sites doubles contact zone and greatly strengthens the bond. Homeodomain Motif  Helix-turn-helix motif surrounded by region initiating key development stages by binding to developmental switch-point genes.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

DNA-Binding Motifs Zinc Finger Motif  Zinc atoms used to coordinate DNA binding. - The more zinc fingers in the cluster, the stronger the protein binds to the DNA. Leucine Zipper Motif  Formed when region on one subunit containing several hydrophobic amino acids interacts with similar region on another subunit.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Controlling Transcription Initiation in Bacteria Repressors are OFF switches  E. coli uses proteins encoded by five-gene cluster to manufacture tryptophan. - All five transcribed as a unit (operon).  When tryptophan is present, the cell shuts off transcription with a tryptophan repressor.  Repressor cannot bind to DNA unless it has first bound to two tryptophan molecules.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Controlling Transcription Initiation in Bacteria Activators are ON switches  Bacterial promoters are sometimes poor binding sites for RNA polymerase, thus genes are not transcribed unless promoter’s binding ability is improved. - Transcriptional activator binds to nearby DNA to improve promoters ability to bind RNA polymerase.  E. coli Catabolite Activator Protein (CAP).

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Fig E. coli Catabolite Activator Protein (CAP).

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Operon – cluster of functionally related genes transcribed into a single mRNA. Lac operon (e. coli) responsible for producing 3 proteins that import the disaccharide, lactose, into the cell to be broken down as glucose and galactose Controlling Transcription Initiation in Bacteria

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Fig Lac Operon

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Ricky w/ here Controlling Transcription Initiation in Bacteria Lac operon possesses two regulatory sites:  Activator - CAP site located adjacent to lac promotor.  Ensures lac genes are transcribed effectively when glucose levels are low.  Repressor - Promoter, located adjacent to operator, determines whether lac genes are transcribed in presence of glucose.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Designing a Complex Gene Control System In eukaryotes, the physical limitation of placing switches around a promoter is overcome by having distant sites exert control over gene transcription.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Designing a Complex Gene Control System Eukaryotic Transcription Factors  For RNA polymerase to successfully bind to a promoter and initiate transcription, a set of proteins (transcription factors) must first assemble on the promoter to guide and stabilize polymerase binding. - Anything that reduces availability of a particular factor will inhibit transcription.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Fig

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Designing a Complex Gene Control System Enhancers  DNA-Binding Domain physically attaches protein to DNA at a specific site, while the Regulatory Domain interacts with other regulatory proteins. - Uncouples regulation from DNA binding, and allows regulatory protein to bind to a specific DNA sequence at one site and exert its regulation over a promoter at another site.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Effect of Chromosome Structure on Gene Regulation Promoter Blocking by Nucleosomes  Histones positioned over promoters block assembly of transcription factor complexes. - Transcription factors appear unable to bind to a promoter packaged in a nucleosome. DNA Methylation  Blocks accidental transcription of “turned off” genes.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Posttranscriptional Control in Eukaryotes Potential Control Points  Processing of Primary Transcript - Exons can be spliced together in different ways, assembling a variety of polypeptides.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies  Transport of Transcript Out of Nucleus  Passage of mRNA transcript across nuclear membrane requires the transcript be recognized by receptors lining pore interiors.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Posttranscriptional Control in Eukaryotes  Selection of Which mRNA’s are Translated - Modification of one or more translation factors affecting translation by ribosomes in cytoplasm.  Translation Repressor Proteins shut down translation by binding to the beginning of the transcript, blocking attachment to the ribosome.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Posttranscriptional Control in Eukaryotes  Selectively Degrading mRNA Transcripts - Stability of mRNA transcripts in cell cytoplasm.  Prokaryotes, half life of 3 minutes.  Eukaryotes, more stable, half life of 10 hours  Regulatory proteins, and growth factors are less stable they have Short half-lives less than 1 hour.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Copyright © McGraw-Hill Companies Permission required for reproduction or display