Lecture 21 Eukaryotic Genes and genomes III 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 phenoty pes 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 Gall 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 as long as glucose is present? In fact, glucose represses a very large number o the preferred carbon source over galactose. Why bother metabolizing galactos 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 Induction of gal genes in yeast It seems reasonable to expect that there is a transcriptional repressor that responds to glucose levels th repressor would be ineffective when glucose is low or absent and effective galctokinas when glucose is present. It also seems activity galactose reasonable that one could isolate trans- acting mutants that fail to repress galactose-induced Gal gene expression in the presence of glucose. However, it acTos a cose turns out that the very fact that glucose es such a large number of different genes made it difficult to GLUCOSE REPRESSION identify such mutantsLecture 21 Eukaryotic Genes and Genomes III 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 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. GLUCOSE REPRESSION It seems reasonable to expect that there is a transcriptional repressor that responds to glucose levels; this repressor would be ineffective when glucose is low or absent, and effective when glucose is present. It also seems reasonable that one could isolate trans￾acting mutants that fail to repress galactose-induced Gal gene expression in the presence of glucose. However, it turns out that the very fact that glucose represses such a large number of different genes made it difficult to identify such mutants. + galactose and gluc nd gluc nd gluc nd glucose