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Lecture 19

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Biology 2581B
Susanne Kohalmi

Genetics Lecture 19 Notes  Judging the effect of a deletion  A silencer region is just like an enhancer except that it has a negative effect instead of a positive effect Transcription levels:  When bound to the silencer, the repressor stops transcription (overrides activation)  A silencer can be located thousands of base pairs away from the gene that it is regulating  Silencer binds a negative transacting factor instead of positive Repressors:  Proteins that binds specific DNA elements and inhibit transcription  Block/destabilize RNA polymerase assembly and movement by binding to DNA sequences very close to the promoter/ farther away (looping)  Block activity of transcription factors/ activators by binding to DNA-binding region of an activator and preventing it from attaching to an enhancer (competition, quenching) – are called corepressor  Repressors have no effect on basal transcription Repressors can act by interfering with the function of activators:  Cis-acting enhancer  all cis-acting elements bound by either activators or repressors are referred to as enhancers, even though some may be repressing transcription  Repressor and activator share a common binding sequence – they are both in competition for that site (mutually exclusive binding)  Quenching is another form of repression  In type 1 quenching, the repressor binds the recognition sequence  the activator can’t bind the recognition element because the repressor is covering up the site that recognizes DNA sequences  Repressor could also block activation domain so that activator is unable to reach the activation domain The ability in genetic analysis is crucial advantage of the use of budding yeast:  The specificity of transcription factors can be altered by other molecules in the cell  Budding yeast can exist in haploid or diploid form  When existing in the haploid form (can be , or a), if you remove nutrients that stimulate it to go into the mating pathway, 2 cells will fuse and become a diploid In “a” cells, 2 repressor binds to enhancers that control the activity of a set of a-determining genes, whose expression would make the cell type “a”  In diploid cells, the same 2 repressor plays another role where expression of the polypeptide a1 occurs – the binding of a1 to the 2 repressor alters the repressor’s DNA-binding specificity such than 2/a1 now binds to enhancers associated with a set of haploid specific genes, repressing the expression of those genes  So in diploid cells, the 2 repressor maintains the diploid state by repressing haploid-specific genes  Can isolate a mutation in a haploid strain (whether it is recessive or dominant) because in the haploid, there is no second copy of a gene Enhancers and enhanceosomes:  Regulatory regions may contain dozens of enhancer elements, each with the ability to bind different activators and repressors with varying affinities  Different sets of transcription factors compete for different enhancers within the regulatory region  Different sets of coactivators and corepressor compete with each other for binding to different activators or repressors  Enhanceosomes – used to describe a multimeric complex of proteins and other small molecules associated with an enhancer element o Changes in a cell’s environment can dispatch signal molecules that cause changes in the balance of transcription factors  leads to assembly of an altered enhanceosome, which re-calibrates gene activity  E.g. the string gene in Drosophila is an example of how transcription can be timed by binding of different transcription factors to coincide with cell proliferation Building a model of a simple transcriptional regulatory network in the budding yeast, Saccharomyces cerevisiae:  Expression of the GAL7, GAL10, and GAL1 genes is required for yeast cells to use galactose as a carbon source  Expression of the genes is galactose inducible  Deletion of cis-acting elements in the upstream regulatory regions defined an enhancer element that acts on all three genes  Loss of function mutations in another gene, GaL4 (GAL4-) prevent galactose from activating expression of the 3 genes  Loss of function mutations in another gene, GAL80 (GAL80-) results in constitutive expression of the 3 genes  Cells bearing loss of function mutations in both GAL4 and GAL80 are unable to activate transcription of the GAL7, GAL10, GAL1 genes in response to galactose  Thus, the GAL80 must affect transcription through its actions in GAL4 (i.e. GAL4 must be more downstream in the pathway) Biochemical evidence helps complete the model:  The GAL4 gene-product has 2 domains: 1. DNA binding domain (recognizes sequence enhancer) 2. Transcriptional activation domain  The GAL80 gene-product binds the GAL4 activation domain (type 2 quenching) 
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