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L ECTURE 20 E UKARYOTIC G ENES AND G ENOMES II In the last lecture we considered the structure of genes in eukaryotic organisms and went on to figure out a way to identify S. cerevisiae genes that are transcriptionally regulated in response to a change in environment. The ability to regulate gene expression in response to environmental cues is a fundamental requirement for all living cells, both prokaryote and eukaryote. We considered how many genes each organism has, about 4,000 for E. coli , 6,000 for yeast and a little over 20,000 for mouse and humans. But only a subset of these genes is actually expressed at any one time in any particular cell. For multicellular organisms this becomes even more apparent…it is obvious that skin cells must be expressing a different set of genes than liver cells, although of course there must be a common set of genes that are expressed in both cell types; these are often called housekeeping genes. There are a number of ways that gene regulation in eukaryotes differs from gene regulation in prokaryotes. Eukaryotic genes are not organized into operons. Eukaryotic regulatory genes are not usually linked to the genes they regulate. Some of the regulatory proteins must ultimately be compartmentalized to the nucleus, even when signaling begins at the cell membrane or in the cytoplasm. Eukaryotic DNA is wrapped around nucleosomes Today we will consider how one can use genetics to begin to dissect the mechanisms by which gene transcription can be regulated. For this we will take the example of the yeast GAL genes in S. cerevisiae . GALACTOSE METABOLISM IN YEAST D-glucose-1-phosphate D-glucose-6-phosphate GLYCOLYSIS Reaction D-galactose D-galactose-1-phosphate UDP-D-galactose UDP-D-glucose Enzyme Gene GAL1 GAL7 GAL10 Galactokinase Galactose transferase Galactose epimerase UDP-glucose Phosphorylase Phosphoglucomutase GAL1 encoded GAL1, GAL7, GAL10 transcription all induced in the presence of glucose. How is this achieved.
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Once a gene has been identified as being inducible under certain inducing conditions, in this case in the presence of galactose, we can begin to dissect the regulatory mechanism by isolating mutants; i.e., mutants that constitutively express the GAL genes even in the absence of galactose, and mutants that have lost the ability to induce the GAL genes in the presence of galactose. If we were studying galactose regulation today we would probably use a lacZ reporter system as we discussed in the last lecture. However, when the Gal regulatory system was fist genetically dissected, it was done by actually measuring the induction of Gal1
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