Class Notes (839,094)
Canada (511,185)
Biochemistry (847)
Chris Brandl (189)
Lecture

Brandl Lecture 2.docx

4 Pages
103 Views

Department
Biochemistry
Course Code
Biochemistry 2280A
Professor
Chris Brandl

This preview shows page 1. Sign up to view the full 4 pages of the document.
Description
Brandl Lecture 2 – Regulation of Gene Expression How are different bacterial promoters transcribed at different levels? - Some sequences have better -10 and -35 sequences  better attraction than others o Will be lower if they cannot attract as well o Not dynamic – cell cannot change -10 and -35 sequences if it wants to up or down regulate that amount of expression - There is more than one sigma factor  each recognizes different promoters o Different sigma factors recognize different sequences o The promoter shown in lecture 1 was for house-keeping sigma/sigma 70 o Most prevalent is housekeeping sigma o Others include sigma 54 in nitrogen metabolism that will turn on metabolism, sigma HS for heat shock – when heat shocked, becomes activated and then turns on in the presence of high temperature  Found in bacteria but not in eukaryotes – conserved for bacteria - Gene specific regulatory proteins – DYNAMIC o Negative regulation – factors repress transcription – trp operon and trp repressor o Positive regulation – factors activate transcription – eukaryotic reduction system Negative regulation – factors inhibiting Trp operon – in E. coli the 5 genes (E, D, B, C, A) for tryptophan biosynthesis are transcribed from a common promoter - Five proteins are required for tryptophan biosynthesis – enzymes are made from this - Trp operon is regulated by concentration of trp in the environment/medium o Trp production is induced when there is little trp in the environment - Q: When is trp operon expressed? A: When there is little tryptophan in cellular environment How is the trp operon regulated by tryptophan? - Trp promoter – typical with +1 start site and -35 and -10 - Special sequence between -35 and -10 sequences called trp operator that binds a protein called the trp repressor o Not true for every promoter but true for trp operon o At high *trp+, trp repressor binds trp o The trp repressor-trp complex binds the operator DNA What happens to transcription? - The trp repressor-trp complex blocks RNA polymerase from the promoter so that no transcription is possible  allosterically inhibits transcription - When *trp+ is low, trp dissociates from trp repressor - The trp repressor no longer binds to the operator - RNA polymerase can access trp promoter. Transcription occurs - When genes are off = high *trp+; when genes are on = low *trp+ What is the structural basis for regulation by trp? - Trp repressor – monomer that does not have catalytic activity o 107 amino acid residues, mostly alpha helices o Alpha helices 4 and 5 make up helix-turn-helix motif that binds the DNA o Works as a dimer o Protein dimer has 2 fold symmetry o 2 alpha Helix 3s contact to allow dimerization o Many DNA binding proteins act as dimers or pseudo-dimers o Helix 5 of each monomer recognizes adjacent major grooves in the operator DNA  Perfect size to fit into major groove  Most base-specific contact made with major groove since minor groove is too small to fit alpha helix - Tryptophan binding induces a conformational change in the trp repressors which allows DNA binding o Alpha helix 5 tilts inwards when trp is low and cannot interact with DNA o Affinity for DNA drops dramatically when protein is not bound by trp o Molecular basis – causes conformational change allowing for binding of DNA - General themes to take from trp operon o Trp repressor is site specific DNA binding protein – operator sequence between -10 and - 35  regulates set of genes that you want regulator to bind to set of genes o There is a binding site for trp repressor within trp promoter o Trp repressor inhibits transcription by blocking access of RNAP to the promoter  Repression works in similar ways by stopping RNAP from getting to promoter o Trp repressor is responsive to environmental signal  Allows it to be turned on and off so it must recognize some signal  Trp specifically represses trp repressor Q: Genetics has been critical in understanding the regulation of the trp operon. Different mutations in the trp repressor can result in? A: the trp operon always being transcribed and the trp operon never being transcribed - Mutations in trp repressor the prevent binding to operator – trp operon always being transcribed - Mutations in the trp repressor that allow it to bind the operator in the absence of trp – trp operon never being transcribed Six differences in transcription between prokaryotes and eukaryotes 1. Three DNA polymerases in eukaryotes but 1 in prokaryotes a. Pol 1 – most rRNA genes b. Pol 2 – mRNA’s c. Pol 3 – tRNAs, 5S rRNA 2. No operons in eukaryotes a. Each gene is transcribed as a single unit i. Monocistronic (one RNA, one protein) – in contrast to polycistronic in bacteria (one protein, multiple RNA) 3. Promoter structure a. Eukaryotes do not have -10 and -35 sequences and no sigma factor b. Promoter recognition is determined by a set of proteins, one of which recognizes a TATA element - TATA box in gene that looks a little like -10 and recruits factors TF2D which has TATA binding
More Less
Unlock Document

Only page 1 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit