Chapter 17 Gene Expression Bacteria Chapter 18 Gene Expression Eukaryotes How do eukaryotes regulate which genes are expressed in what amounts and when Learning Objectives By the end of this class you should be able to 1 Explain how the metabolism of lactose in e coli is an example of gene expression regulation 2 Describe the six sites of gene expression regulation in eukaryotes 3 Comprehend how the cell packages DNA and how this packaging affects expression 4 Describe how the gene expression is regulated by physical contact between regulatory proteins and specific regulatory sites in DNA The lac Operon 1 The lacZ lacY and lacA genes are adjacent and transcribed into mRNA 2 The repressor is a protein encoded by lacI that binds to DNA operator and prevents lacZ Y and A 3 The inducer lactose binds to the repressor changing its shape so that it off the DNA The lac Operon lacI X X X 1 The lacZ lacY and lacA genes are adjacent and transcribed into mRNA 2 The repressor is a protein encoded by lacI that binds to DNA operator and prevents lacZ Y and A 3 The inducer lactose binds to the repressor changing its shape so that it off the DNA But where does glucose fit in Transcription of the lac operon is drastically when glucose is present even when lactose is available to induce galactosidase expression Lactose Metabolism Model System Glucose is e coli s preferred source but when glucose levels are low it will use lactose If glucose and lactose are present glucose the expression of galactosidase Mechanisms of Positive Control hydrolysis Glucose The of lactose into glucose and galactose is an example of catabolism Catabolism repression is a form of feedback inhibition where glucose is the decreasing the transcription of the lac operon Catabolite Repression Presence of only lactose Presence of glucose and lactose CAP Catabolite activator protein The CAP protein binds to the CAP binding site transcription of the lac operon Mechanism of Control Negative control occurs when a regulatory protein binds to DNA and shuts down transcription Positive control occurs when a regulatory protein binds to DNA and triggers transcription Where does glucose fit in We need cAMP to bind to CAP for transcription Glucose regulates cAMP levels Glucose cAMP Glucose cAMP cAMP lacZ transcription Why was this important Many genes and operons were shown to be under negative and positive control meaning these findings were general Also showed that gene expression is regulated by physical contact between regulatory proteins and specific regulatory sites in DNA Mechanisms of Gene Regulation Eukaryotes Nucleus Cytoplasm Chromatin Remodeling DNA wraps almost 2 times around a group of 8 and a histone called H1 seals it The are packed into 30 nm fibers then attached to scaffold proteins which can be highly condensed during cell division What does this look like in the cell Think Pair Share What do you think these histone proteins are made up of to interact so closely with DNA Be specific What effect might this packaging have on gene expression Must chromatin be relaxed for RNA polymerase to bind to a promoter Two pieces of evidence support this hypothesis Found DNase would find and cut up only those genes in cells which were actively expressed Must chromatin be relaxed for RNA polymerase to bind to a promoter Two pieces of evidence support this hypothesis Found DNase would find and cut up only those genes in cells which were actively expressed Must chromatin be relaxed for RNA polymerase to bind to a promoter Two pieces of evidence support this hypothesis Second mutants that do not produce normal histones produces high transcription of genes that are normally never transcribed How is chromatin altered Chromatin Remodeling Some work has shown that modifications of histones play an important role Acetylation of histones by histone acetyl HATs is usually associated with positive control Histone deacetylases HDACs remove acetyl groups causing it to condense Initiating Transcription Most promoters like the conserved TATA box sequence are bound by TATA binding TBPs Many genes also have promoter proximal elements that are unique to specific genes unlike the promoter region TATA Box TATA Binding Protein TATA Binding Proteins The binding of the TATA binding proteins tell the cell What to transcribe our DNA in Where to transcription Protein binding at both places
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