Post-transcriptional regulation by Tryptophan biosynthesis and regulatory RNA Flashcards
when does Post-transcriptional Regulation occur?
- During transcription by attenuation.
- initiation of translation– Trans-encoded Regulatory small RNAs and Cis-encoded antisense RNA
- During translation by translational coupling
describe Transcriptional Attenuation
- controls gene expression just after transcriptional initiation as RNA complex progresses through 5
-untranslated region (5-UTR) - 5`UTR of some genes can form terminator structure before the functional protein coding sequence
how does tryptophan Post-transcriptionally regulate?
- tryptophan operon can be switched off when tryptophan binds to trp repressor protein.
- tryptophan is a co-reppressor.
- structural proteins not transcribed.
explain transcriptional attenuation in the trp operon
- the leader sequence contains 4 palindromic sequences.
- at high [trp] when 1-2 and 3-4 bind theres a intrinsic terminator of transcription that binds RNA pol.
- low [trp] 2-3 hairpin transcription continues.
When levels of tryptophan are low?
- Ribosome encounters codons encoding tryptophan
- low levels of tryptophan means low levels of tryptophanyl-tRNA •Ribosome temporarily stalls
- Ribosome not open 2-3 hairpin loop
- RNAP progresses thru past stem 4 and into structural genes
Why multiple regulatory controls?
• Control of tryptophan biosynthesis:
• Tryptophan regulatory molecule – TrpR- initiation control
– Feedback control
• tRNATRP regulatory molecule – Attenuation
• Conditionsoccurwhenseparateeventscould affect levels of tryptophan and tRNATRP separately
When levels of tryptophan are high?
- Ribosome encounters codons encoding tryptophan
- high [tryptophan] means high [tryptophanyl-tRNA]
- Ribosome opens 2-3 hairpin loop
- 3-4 hairpin termination loop forms and transcription terminates
Other examples of attenuation?
- Nutrient biosynthesis and import operons: – Amino acids, metal ions, nucleic acids
- tRNA synthesase genes-e.g.phe,thr
what are the two types of non-coding RNA?
- cis-encoded antisense RNA
2. Trans-encoded small regulatory RNA
what are small RNAs?
- Un-translated RNA molecule 50-200 nucleotides
- encoded by cis ORF
- Expressed under specific environmental conditions
explain Positive regulation by regulatory RNA
- Prevent formation of structure inhibiting translation
- Ensure processing of 5` end of mRNA
- Stabilise mRNA by blocking ribonuclease access
what is Hfq- RNA binding protein?
an RNA chaperone
properties of Hfq protein
- Highly abundant in Gram-negatives
• Highly conserved in prokaryotes
• Hfq has high similarity to eukaryotic Sm-like proteins
• Acts as a hexamer-forms ring-likes tructure with central pore
• Hfq mutants have pleiotropic effects
what does Hfq- RNA binding protein do?
- Stabilises RNAs
- Required for sRNA to interact with target mRNA
- Modulate stability of mRNAs
- Most Hfq regulation is negative as interaction with sRNA results in decreased translation or degradation of mRNA
Advantages of RNA regulators in stress situations
- Reduced metabolic costs
- Provides additional levels of control
- Act at post-transcriptional level so ensure fast response
- Unique regulatory properties- coupled degradation of mRNA:sRNA pairs
- Regulate target at different levels-protein and RNA, reduces leakiness and alters dynamics
- Evolutionary advantages –limited base pairing between target and regulatory RNAs allow for flexibility in evolving new targets
what does Small RNA-micF do?
- micF regulates expression of OmpF porin
- Small RNA thought to provide another point where other regulators can affect porin expression
- Larger porin size make small molecules that are detrimental to cell more likely to enter
- Not necessarily change osmolarity detected by EnvZ
- examples–antibiotics,bilesalts,salicylate, oxygen stress, EtOH, temperature
Genetic evidence of post transcriptional control
- increase in micF expression reduces ompF expression
- but deletion of micF does not greatly affect ompF expression
- Whereas deletion of a transcriptional repressor would result in constitutive expression of ompF
how is micF regulated?
- micF transcription is regulated in response to environmental signals
- micF promoter has number of binding sites for transcriptional activators
- micFtranscriptioninducedby:
- SoxRS - oxidative stress
- MarA–weakacids
- Rob–cationicpeptideantibiotics
- OmpR–highosmoticstress
- Also induced by temperature and rich nutrient growth conditions
give an example of translational coupling
In OmpR-EnvZ system:
– 1000 OmpR molecules/cell
– 10 EnvZ molecules/cell
– co-transcribed from one promoter – one transcript
• poor re-initiation of translation leads to polar effect on expression
• fine control of expression levels but fixed
how bacterial gene expression can be controlled following initiation of transcription?
– Attenuation
– Antisense RNA
– Translational coupling
two additional mechanisms bacteria can use to respond to environmental change?
Attenuation/Antisense
what is micF?
- gene transcribed divergently from ompC
- 93bp RNA
- ~70% complementary to RBS of ompF mRNA
- Model:micF/ompF RNAs form a duplex
- Duplex shields ompF RBS and AUG
- Inhibits translation initiation
