Lecture 5 - Termination of transcription Flashcards
Outline Intrinsinc/Rho-INDEPENDANT Termination
The formation of stem-loop structure in mRNA causes dissociation of mRNA owing to weak dA-rU base pairing.
This is simultaneous to the DNA bubble closing up and RNA polymerase pausing.
What is a typical terminator made of base-wise?
Poly U at 3’ terminus
Dyad symmetry
What does RNA-DNA interaction rely on?
G=C and A=U, A=T composition and H-bonding
dA=rU is the weakest base pairing possible
No protein assistance required
What is Rho-DEPENDANT termination?
No Oligo-U’s at 3’ end of stem loop like independent.
Uses a Rho protein that uses ATP+H20->ADP+Pi to terminate transcription.
Rho protein:
Mr = 6x46,000
Binds to 72 ribonucleotides of RNA
Helicase which slides along nascent RNA and pulls it out of transcription bubble.
The length of mRNA produced is proportional to the time Rho protein is present at start of synthesis.
What are control mechanisms involving transcription termination?
Anti-terminator proteins:
Page Lambda N and Q proteins bind at specific sites on DNA and allow transcription to continue to next lytic stages
Attenuation:
Control of amino acid biosynthesis
Outline tryptophan biosynthesis
Chorismic acid + L-Glutamine e: trp E Anthreanilic acid + PRPP e: trp D Phosphoriosyl anthranilic acid e: trp C CDPR e: trp C Indole-3-glycerol phosphate e: trp B/A Indole + L-Serine e: trp B/A L-tryptophan
Where is the Leader and Attenuator in trp operon found?
Before structural genes
What does the deletion of attenuator do?
Yanofsky
Increased expression of trp operon
Outline attenuation mechanism
The ultilisation of ribosomes to control translation
Absence of protein synthesis:
A stem loop between 1 and 2
A secondary stem loop between 3 and 4, this causes transcriptional termination
Protein synthesis with limiting trp:
Ribosomes stall at trp codons, blocking formation of 1:2 loop.
2:3 Loop is formed
3:4 is prevented, transcription continues
Protein synthesis with sufficient trp:
Ribosomes blocks formation of 1:2 and 2:3 loop formations.
Hence forms 3:4 loop which causes translational termination
What is fine control?
Proportion of long transcripts is inversely dependant on [TRP].
e.g. absence of trp will prevent formation of 3:4 structure (termination)
Outline the bases of trp operon
UGG UGG
Outline the bases of phe operon
UUU UUU UUC UUC UUU UUU UUC
Outline the bases of his operon
CAC CAC CAU CAU CAC CAU CAU
What is thr regulated by? (fine control)
Isoleucine and threonine.
What is ilv regulated by? (fine control)
leucine, valine, isoleucine
