Exam 3 Lecture 22 Flashcards
(36 cards)
Where is the attenuator region of the Trp operon?
At the end of the regulatory region in between the operator and the first Trp gene.
What does the attenuator in Trp do?
Adds an extra level of regulation to fine-tune down-regulation depending on which regions of it bind together. Functions on the newly transcribed mRNA.
What 10 parts make up the Trp operon and what order are they in?
- Promoter for TrpR
- trpR the regulatory gene for the aporepressor
- Promoter for the main gene sequence
- Operator Region
- Leader Region trpL- attenuator is in the middle of it
- trpE gene
- trpD gene
- trpC gene
- trpB gene
- trpA gene
How does an attenuator work?
Translation of the trp gene starts and the attention sequence is made first. A ribosome can hop on and get ready to start transcription but the first attenuation sequence of the mRNA has 4 regions that can bind to each other form stem-loop complexes. Depending on what regions bind with what the response is different.
What are the 3 responses of the attenuation of the trp operon?
- Pause to allow ribosome time to start translation
- Antiterminator loop pauses ribosome/translation allows transcription
- Termination
What happens when attenuator regions 1 and 2 bind
Pause loop forms
What happens when trp attenuator regions 2 and 3 bind?
Anti-terminator loop
What happens when trp atentuator regions 3 and 4 bind?
Terminator loop is formed
Which trp attenuator loop forms first?
Regions 1 and 2 to pause transcription till the ribosome gets on causes the loop to release so transcription can continue.
When does trp attenuator regions 2 and 3 to bind and why?
In times of low trp when there aren’t enough tRNA with trp on them, the ribosome will stall at an area of T’s on the mRNA in region one. There isn’t a tRNA-T to keep going! then regions 2 and 3 will bind since it is more energetically favorable than the 3 and 4 binding.
2 and 3 is the anti-terminator loop and it allows the mRNA transcription to continue and transcribe trpE but stops translation.
What happens in high levels of trp to the attenuator region if for some reason high levels of trp didn’t already bind to the aporepressor to make a holorepressor and bind to the operator to stop transcription?
Ribosome hops on translates through the trp area in region 1 gets to region 2 and while covering region 2 gets to a stop codon allowing regions 3 and 4 to bind- the termination loop.
How does the trp termination attenuator loop work?
The termination loop is G-C rich followed by a bunch of UUUU acts and an intrinsic transcription stop. It also binds to polymerase causing it to release before reaching trpE and no trp mRNA to be made.
3 main types of regulatory RNAs?
- Riboswitches
- asRNA (antisense made from the antisense DNA strand while the sense mRNA is being made)
- sRNA (small) encoded in regions between genes
What do regulatory RNAs do?
Regulate mRNA translation sometimes transcription by
- interacting with mRNA
- and proteins
What does a riboswitch do?
Riboswitches control transcription or translation via stem-loop and/or ligand binding.
Usually at the 5’ UTR
Think trp attenuator
Example of a riboswitch?
The trp attenuator is a riboswitch
asRNA are how long?
700-3000 nt long
Where are asRNA ‘s made from?
The antisense DNA strand while the sense mRNA is being transcribed.
Regulate only that gene or operon
Two main ways asRNA works?
- RNA polymerase will get on antisense strand and collide with sense RNA polymerase
- The antisense strand is transcribed without a collision and goes on to bind to the sense mRNA and interfere during either transcription or translation.
How does asRNA work to interfere with transcription (other than polymerase collision)?
Binds tightly to mRNA to being made and can stop transcription like an attenuator does or an intrinsic transcription stop.
Happens on plasmids often.
Two other ways asRNAs work?
- Endonuclease activity either blocking or tagging it for destruction
- Translational blocking either directly or indirectly
Specific example of asRNA?
The S-box or SAM (s-adenosylmethionine) riboswitch
Antisense RNA and a riboswitch both at the same time!!
Also uses transcription interference RNA poly collision!
All three concepts in one!
How does the SAM/S-box asRNA work?
RNA poly get on antisense DNA of ubiG-mccB-mccA operon
The first bit of the asRNA codes the switch
When not bound to anything aka low levels of ligand SAM anti-terminator loops are made and transcription continues causing and RNA polymerase collision with sense strand RNA poly and termination of transcription.
BUT when SAM is around it binds and causes a termination loop allowing sense RNA strand to be made.
What are sRNA’s? How long?
Small RNAs
100-200 nts
