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Unformatted text preview: Lecture 18 Lecture 18 Lecture 18 Lecture 18 Lecture 18 In the preceding examples of bacterial gene regulation, we have used known regulatory mechanisms to see how mutations in different elements of the system would behave in dominance tests and cis/trans tests. However, one is often trying to learn about a new operon and is therefore faced with the problem of deducing mechanism from the behavior of mutants. The steps to analyzing a new operon are as follows: 1) Isolate mutants that affect regulation. These could be either constitutive or uninducible. The most common regulatory mutations are recessive loss of function mutants in trans-acting factors. This is because there are usually many more ways to disrupt the function a gene than there are ways to make a dominant mutation. Promoter, operator, and initiator sites are usually much shorter than genes encoding proteins and these sites present much smaller targets for mutation. 2) Check to see whether the mutation is recessive and trans-acting (most will be). If the mutation is constitutive then it is likely in the gene for a repressor. If the mutation is uninducible then it is likely in the gene for an activator. repressor activator enzyme + enzyme Although loss of function mutations in genes for repressors or activators are generally the most common type of regulatory mutation, the table below will help you to interpret mutations in sites or more complicated mutations in proteins. Type of Mutation Phenotype Dominant/Recessive Cis/Trans-acting repressor constitutive recessive trans-acting activator uninducible recessive trans-acting operator constitutive dominant cis-acting promoter uninducible recessive cis-acting repressor-d-d-d-d-d constitutive dominant trans-acting or activator s repressor s uninducible dominant trans-acting or activator- d Regulatory Pathways Regulatory Pathways Regulatory Pathways Regulatory Pathways Regulatory Pathways So far we have been considering simple regulatory systems with either a single repressor (Lac) or a single activator (Mal). Often genes are regulated by a more complicated set of regulatory steps, which together can be thought of as a regulatory pathway regulatory pathway regulatory pathway regulatory pathway regulatory pathway . Although there are good methods that can be used to determine the order of steps in a regulatory pathway (as will be discussed shortly), it is usually difficult at first to tell whether a given component identified by mutation is acting directly on the DNA of the regulated...
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