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The Genetics of Viruses and Prokaryotes. 13 The Genetics of Viruses and Prokaryotes. 13.1 How Do Viruses Reproduce and Transmit Genes? 13.2 How Is Gene Expression Regulated in Viruses? 13.3 How Do Prokaryotes Exchange Genes? 13.4 How Is Gene Expression Regulated in Prokaryotes?
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13 The Genetics of Viruses and Prokaryotes • 13.1 How Do Viruses Reproduce and Transmit Genes? • 13.2 How Is Gene Expression Regulated in Viruses? • 13.3 How Do Prokaryotes Exchange Genes? • 13.4 How Is Gene Expression Regulated in Prokaryotes? • 13.5 What Have We Learned from the Sequencing of Prokaryotic Genomes?
13.1 How Do Viruses Reproduce and Transmit Genes? Viruses are acellular. Most are composed only of nucleic acids and some proteins. Viruses do not: Regulate transport of materials into and out of themselves Perform any metabolic functions
13.4 How Is Gene Expression Regulated in Prokaryotes? Prokaryotes make proteins only when they are needed. Regulation of protein synthesis is usually done by transcriptional regulation.
13.4 How Is Gene Expression Regulated in Prokaryotes? Example: E. coli in the intestine has a variable food source. If lactose is present, three enzymes are required for uptake and metabolism of the lactose. If grown on medium with no lactose, levels of these proteins are very low.
13.4 How Is Gene Expression Regulated in Prokaryotes? Inducers are compounds that stimulate synthesis of a protein (e.g., lactose). The proteins produced are inducible proteins. Proteins made all the time at a constant rate are constitutive.
Figure 13.16 An Inducer Stimulates the Synthesis of an Enzyme
13.4 How Is Gene Expression Regulated in Prokaryotes? The rate of a metabolic pathway can be regulated by: Allosteric regulation of enzyme activity Regulation of protein synthesis—slower, but produces greater energy savings
13.4 How Is Gene Expression Regulated in Prokaryotes? Structural genes specify primary protein structure—the amino acid sequence. The three structural genes for lactose enzymes are adjacent on the chromosome, share a promoter, and are transcribed together.
13.4 How Is Gene Expression Regulated in Prokaryotes? Prokaryotes shut down transcription by placing an obstacle—the operator— between the promoter and the structural gene. The operator binds to a protein called a repressor—blocks transcription of mRNA.
13.4 How Is Gene Expression Regulated in Prokaryotes? Operon: the whole unit—promoter, operator, and one or more structural genes. Operon containing genes for lactose metabolism: lac operon. ANIMATION
13.4 How Is Gene Expression Regulated in Prokaryotes? Repressor protein has two binding sites: one for the operator, one for inducer (lactose). Binding to inducer changes the shape of repressor, allows promoter to bind RNA polymerase. When lactose concentration drops, inducers separate from repressors—repressor again binds operator, transcriptions stops.
13.4 How Is Gene Expression Regulated in Prokaryotes? Repressor proteins are encoded by regulatory genes. Gene for repressor in lac operon—i gene. It also has a promoter pi, but no operator. The repressor is constitutive.
13.4 How Is Gene Expression Regulated in Prokaryotes? A protein is repressible if synthesis can be turned off by a biochemical cue (e.g., ample supply of that protein). The trp operon controls synthesis of tryptophan—it is a repressible system. The repressor must first bind with a corepressor, in this case tryptophan.
13.4 How Is Gene Expression Regulated in Prokaryotes? Inducible systems: Substrate of a metabolic pathway (inducer) interacts with a regulatory protein (repressor)—repressor cannot bind to operator—allowing transcription. Repressible system: Product of a metabolic pathway (corepressor) interacts with a regulatory protein (repressor) allowing it to bind to operator, blocking transcription.
13.4 How Is Gene Expression Regulated in Prokaryotes? Inducible systems control catabolic pathways (turned on when substrate is present). Repressible systems control anabolic pathways (turned on when product is not present).
13.4 How Is Gene Expression Regulated in Prokaryotes? lac operon can increase efficiency of the promoter. A regulatory protein CRP binds cAMP—this complex binds to DNA just upstream of promoter. Allows more efficient binding of RNA polymerase to the promoter.
Figure 13.21 Catabolite Repression Regulates the lac Operon (Part 1)
Figure 13.21 Catabolite Repression Regulates the lac Operon (Part 2)
13.4 How Is Gene Expression Regulated in Prokaryotes? Catabolite repression: The presence of glucose lowers the cell concentration of cAMP, thus less CRP binding to promoter, resulting in less efficient transcription.
13.4 How Is Gene Expression Regulated in Prokaryotes? The lac and trp systems are negative control of transcription—the regulatory protein prevents transcription. Catabolite repression is positive control—the regulatory CRP-cAMP complex activates transcription.
