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Cis - Gal1 gene as an example We have considered the fact...

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Cis-acting sequences In the last lecture we considered a classic case of how genetic analysis could be used to dissect a regulatory mechanism. This analysis was contingent upon having “clean” phenotypes associated with the isolated mutants; e.g., mutations in the Gal80 gene produce a phenotype of constitutive Gal1 expression. However, it is sometimes very difficult to identify regulatory proteins by isolating mutants, because regulators that influence the expression of a wide variety of genes might be essential (i.e., mutations in these could be lethal), or their mutant phenotypes may be extremely complex and difficult to interpret. One solution to this has been to work backwards from the cis-acting promoter sequences for particular genes to identifying the proteins that bind to them. Let’s take the
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Unformatted text preview: Gal1 gene as an example. We have considered the fact that in the presence of galactose the Gal1 gene is transcriptionally upregulated (along with other Gal genes). What I haven’t told you is the fact that if glucose is present in addition to galactose, the induction of the Gal genes simply does not occur! This is known as glucose repression . This makes physiological sense because glucose is a more efficient energy source for yeast, and is therefore the preferred carbon source over galactose. Why bother metabolizing galactose as long as glucose is present? In fact, glucose represses a very large number of genes whose products metabolize a wide range of carbon sources (sucrose, maltose, galactose etc) that are less energy efficient than glucose, as well as repressing a whole host of other genes....
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