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Transcription Termination

Announcements:

• Homework #7 is graded, and the key is online
• Homework #8 (chapter 10) is online, and is due at 5 pm next Monday, Nov 6
• Today's lab is time-consuming--it will be important to be organized and efficient
• Midterm exam II is coming up sooner than you might think--Monday, 11/13/00 (which means that this would be a good time to start looking back over chapters 9 and 10 and their learning objectives)

Learning objectives for Wed 11/1/00

Students should be able to…

1. describe the mechanism of rho-independent prokaryotic transcription termination, as far as it is understood, using the trp operon as an example
2. describe the mechanism of attenuation of the trp operon: how attenuation works to prevent transcription of the full-length trp mRNA, and how this attenuation is regulated by tryptophan. Explain why this particular regulatory mechanism is specific to prokaryotes.
3. compare and contrast attenuation and anti-termination (e.g. by lambda phage N protein) as mechanisms of control of transcriptional elongation

Outline:

• most prokaryotic transcriptional termination appears to rely on sequences in the nascent mRNA that form a step-loop secondary structure followed by a run of U's
• RNAs can have alternate stem-loop secondary structures--e.g. a stretch of nucleotides near the beginning of the trp operon mRNA in E. coli can participate in either of two alternate step-loops
• alternate RNA secondary structures, together with the coupling of transcription and translation in prokaryotes, provides the basis for attenuation as a mechanism of regulation of gene expression
• attenuation at the trp operon, and its regulation by tryptophan
• sometimes prokaryotic transcription termination uses a protein called rho, which binds to the nascent RNA and actively interferes with RNA polymerase
• anti-termination depends on RNA-binding proteins that interfere with termination

General outline for Chapter 11:

• Wed 11/1: transcription termination mechanisms and regulation
• Fri 11/3: RNA processing: 5' capping, 3' cleavage and poly-adenylation, intro to splicing
• Mon 11/6: RNA splicing mechanisms and regulation, Drosophila sex determination
• Wed 11/8: Nuclear transport mechanisms and regulation
• Fri 11/10: discussion of outside reading (Drosophila sex determination)

Chapter 11 sections we'll skip:

• structure/function relationship of RNA binding proteins
• RNA editing and localization to specific regions in the cytoplasm
• processing of rRNA and tRNA
• self-splicing introns

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