Module 6: Reg. of Gene Expression (Post-Initiation Reg.) Flashcards
At what point POST-TRANSCRIPTION (initiation) can protein synthesis be controlled?
1) Inhibition of transcription AFTER initiation (attenuation)
2) Enzymatic degradation of mRNA
3) Inhibition of mRNA translation
4) Post-translational modification of proteins
What are two main regulatory mechanisms of post-transcriptional regulation?
1) Regulatory RNAs (sRNAs)
2) Secondary mRNA structure
–> Including Riboswitches
Aside from mRNA coding DNA regions, all genomes contain regions of DNA…
Regions of DNA that encode for NON-TRANSLATED RNA
Non-Translated RNA
RNAs that do not directly lead to protein synthesis!
–> Do not undergo translation!
What are the types of non-translated RNA?
1) tRNA (transfer RNA)
2) rRNA (ribosomal RNA)
3) sRNA (small noncoding RNA)
sRNA
(define)
= “small non-coding RNA”
–> Molecules around 50-400 nucleotides in length that operate by a variety of methods BUT typically are involved in gene regulation by interacting with existing mRNAs
What do sRNAs do generally?
Interact with existing or forming mRNAs causing alterations that impact the mRNA’s completion of transcription or initiation of translation
–> Play a role in gene regulation!
What are sRNAs AKA?
1) miRNA = micro RNA
2) siRNA = Small interfering RNA
3) asRNA = Antisense RNA
Interactions of sRNA with mRNA can…
1) Prevent the completion of transcription
2) Block translation of existing mRNA transcripts
–> Messes with the expression of genes!
What is one example of sRNA in action?
The interaction of an sRNA (dsRA) with an mRNA transcript encoding for sigma-38 (RpoS)!
(Involved when low temperature occurs)
What is wrong with the mRNA produced from the sigma-38 (RpoS) gene?
The mRNA has a hairpin stem-loop secondary structure in which:
1) The ribosomal binding site and start codon are inaccessible
= Ribosome CANT bind and translation cannot occur!
2) RNAse cut sites that if cleaved, break the entire molecule apart
How does sRNA play a role in “fixing” the issues with the sigma-38 (RpoS) mRNA?
A specific sRNA called “DsrA” binds to the RpoS mRNA (along with the Hfq binding protein) to cause a structural change that breaks open the hairpin stem-loop strucutre
== ribosomal binding site becomes active!
AND
RNAse cut site no longer cleaves the entire molecule but just the non-coding leader end!
What sRNA molecule is involved with the sigma-38 (RpoS) mRNA during LOW temperature?
DsrA
What sRNA molecule is involved with the sigma-38 (RpoS) mRNA during cell surface stress?
RprA
DsrA
an 85 nucleotide asRNA molecule that is produced and accumulates within the cell when low temperature conditions occur
–> binds to leader sequence of RpoS mRNA
When does DsrA accumulate in the cell?
During very LOW temperature conditions!
What does DsrA control within the rpoS gene regulation?
Controls whether or not translation occurs!
DsrA accumulated = can bind to rpoS mRNA and translation occurs
DsrA NOT accumulated = doesn’t bind to rpoS mRNA, no translation can occur, and mRNA transcript is degraded!
DsrA present in cell: Process
DsrA present:
1) RpoS mRNA is produced
2) DsrA + Hfq protein bind to the RpoS mRNA
3) Binding triggers secondary structural change of the mRNA = ribosomal site becomes accessible
4) Ribosome binds and begins to translate the RpoS mRNA!!!
== RpoS (sigma factor) is produced; goes to bind to RNA polymerase to direct it to the regulon involved in cold response!
DsrA Absent in cell: Process
DsrA Absent:
1) RpoS mRNA is produced
2) NO accumulation of DsrA = no binding to the mRNA
3) Without DsrA binding, the mRNA CANT be translated
4) mRNA gets cleaved by RNAses and degrades!
=== no translation of rpoS leading to no production of RpoS leading to no activation of the regulon involved in cold response!
Hfq
RNA binding protein that promotes binding of sRNA to mRNA!
In RpoS mRNA, what is causing the formation of the hairpin stem-loop?
The LEADER SEQUENCE of the mRNA
== basepairs with downstream region of the RNA molecule to form the inhibitory secondary structure
What does DsrA need in order to bind to mRNA leader seqeuence?
Hfq!
RprA
A 105 nucleotide asRNA that functions the same as DsrA BUT accumulates under cell surface stress (rather than low temperature)
Leader Sequence
A non-coding region UPSTREAM of the ribosomal binding site (5’ end)