Chapter 17 Gene Expression Bacteria How do bacteria regulate which genes are expressed in what amounts and when Cell Bio Fair 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 what the mechanisms of negative and positive control 3 Comprehend how complex regulation can be even in a small organism like e coli 4 Describe how the gene expression is regulated by physical contact between regulatory proteins and specific regulatory sites in DNA Lactose Metabolism REVIEW is e coli s preferred carbon source but when glucose levels are low it will use lactose This requires the enzyme galactosidase to break lactose down into and In the presence of glucose lactose is not used as a carbon source because the gene products proteins are not made Lactose Metabolism Mutants Constitutive mutant Expression is on all the time Lactose Metabolism Mutants inhibits transcription of lacZ and lacY lacI acts negatively towards transcription of lacZ and lacY 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 The Hypothesis of Negative Control Defining the repression of the repressor How do we tell which one is correct Defining the repression of the repressor How do we tell which one is correct Assay look for the presence of the lacI protein in the presence of only lactose The lac Operon 1 The lacZ lacA genes are adjacent and transcribed into one mRNA 2 The is a protein encoded by lacI that binds to DNA operator and prevents transcription lacZ Y and A 3 The inducer binds to the repressor changing its shape so that it drops off the DNA But where does glucose fit in Transcription of the lac operon is drastically reduced when glucose is present even when lactose is available to induce galactosidase expression Lactose Metabolism Model System Glucose is e coli s preferred carbon source but when glucose levels are low it will use lactose If glucose and lactose are present prevents the expression of galactosidase Mechanisms of Positive Control hydrolysis Glucose The hydrolysis of lactose into glucose and galactose is an example of catabolism Catabolism repression is a form of feedback inhibition where glucose is the catabolite decreasing the transcription of the lac operon Catabolite Repression CAP Catabolite activator protein The CAP protein binds to the CAP binding site triggering transcription of the lac operon Mechanism of Control Negative control occurs when a regulatory protein binds to and shuts down transcription Positive control occurs when a regulatory protein binds to DNA and triggers transcription Where does glucose fit in Glucose regulates cAMP levels Glucose cAMP Glucose cAMP 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
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