cid 127 Gene expression converting information archived in DNA into molecules that do things in the cell An Overview of Gene Regulation and Information Flow Cells must use resources efficiently if they want to survive and reproduce Chapter 18 Control of Gene Expression in Bacteria Mechanisms of Regulation DNA mRNA protein activated protein gene expression can be controlled at any of these steps cid 127 Mechanisms for gene regulation Transcriptional control affects RNA polymerases ability to bind to a promoter and initiate transcription Slow but efficient saves the most energy for the cell because it stops the process at the earliest possible point Translational control regulatory enzymes alter the length of time mRNA survives or initiation and elongation Allows for rapid changes in amounts of protein because mRNA is already present Post Translational control activating or inactivating the protein cid 127 Most rapid response only one step is needed to activate an existing protein energetically inefficient There is a trade off between speed of response and conservation of ATP amino acids and other resources Some genes are transcribed constitutively all the time Metabolizing Lactose A Model System An inducer is a molecule that triggers transcription of a specific gene Identifying Regulated Genes To find mutants associated with a particular trait researchers must use two steps cid 127 Generate a large population of mutations at random locations in the genome cid 127 Mutagens X rays UV light or chemicals that damage DNA and increase mutation rates Screen the treated individuals for mutants with defects in the process or biochemical pathway in question See page 339 for replica plating process Several Genes are Involved in Lactose Metabolism Cells that are abnormal because they produce a product at all times are called constitutive mutants Negative Control of Transcription There are two ways that genes can be regulated Positive Control activator binds to DNA and triggers transcription Negative Control repressor binds to DNA and shuts down transcription The Operon Method cid 127 Operon set of coordinately regulated genes that are transcribed together into one mRNA Cotranscription genes are adjacent and are transcribed together into one mRNA initiated from the single promoter cid 127 Operator the section of DNA which a repressor binds to Allosteric regulation inducer binds to repressor causing it to change shape and function How Does glucose Regulate the lac Operon Inducer exclusion lets cells use one type of sugar even when others are present Why Has the lac Operon Model Been So Important cid 127 Gene expression is regulated by physical contact between regulatory proteins and specific regulatory sites in DNA Repressor protein is always present it is transcribed and translated constitutively Doesn t require changes in transcription or translation function of existing repressors is altered Positive Control of Transcription Global Gene Regulation Activator protein binds to a regulatory sequence in DNA when genes are turned on cid 127 Global gene regulation is the coordinated regulation of many genes Regulon set of separate genes or operons that contain the same regulatory sequences and are controlled by a single type of regulatory proteins Regulons can be under both negative and positive control Interactions among protein regulators and the DNA sequence allow fine tuned control over gene expression cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127
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