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Medical and Molecular Genetics Lecture 7 Regulation of Gene Expression 1) Define the terms cis-acting and trans-acting and give examples of cis-acting elements and trans-acting factors responsible for gene regulation. Cis-acting elements are the DNA sequences that participate in regulating genes. Trans-acting factors are the proteins that interact with cis-acting elements. These elements and factors working together serve to induce, repress, de-induce, or de-repress the expression of a gene. There are two major types of cis-acting elements: promoters and regulatory elements (response elements and enhancers). Trans-acting factors bind to either cis-acting elements, other factors, or RNA polymerase. 2) Distinguish the differences between a promoter, a response element, and an enhancer. Promoters are the sites where RNA polymerase must bind to the DNA in order to initiate transcription. Response elements are DNA sequences that coordinately regulate the expression of groups of genes and are located a few hundred base pairs upstream of the promoter. Enhancers are basically the same as response elements, but they differ from a regular response element in that they can function either upstream, within, or downstream of a gene. 3) List the types of domains that a trans-acting factor may need for activity. Transacting factors may contain multiple domains required for activity: a DNA-binding domain, a ligand-binding domain, phosphorylation sites, an oligomerization domain, and a transcription-activating domain. 4) Diagram how steroid receptors regulate transcription. Steroid receptors function as homodimers and bind to response elements that consist of inverted repeat sequences separated by 3 base pairs. Inactive steroid receptors are bound to a dimeric protein called hsp 90 in the cytoplasm. Upon binding of the steroid, the receptor dissociates from the hsp 90. The hormone bound receptor is then transported into the nucleus where it then binds as a homodimer to its target response element. The receptor’s transcriptional activation domain interacts with the transcription machinery and induces or represses transcription. Once concentration of the hormone drops, the receptor dissociates and is no longer capable of binding the hormone. It must be transported back to the cytosol and re-associate with hsp 90 before it can bind again. 5) Explain how alternative RNA processing and repression of translation affect gene expression. Alternative RNA processing alters gene expression to achieve tissue specific expression of certain proteins. Alternative polyadenylation sites and/or alternative splice sites are frequencly used to generate similar proteins with specific tissue functions. Repression of translation is accomplished through a sequence of the Iron Responsive Element of the transferring receptor mRNA, which is also found in the 5’ end of apoferritin mRNA. However, since it binds at the 5’ end it represses translation of the apoferritin mRNA until high concentrations of iron bind the repressor protein and induce it to dissociate from the mRNA.