MBB 267 Week 11: Mitchel 11

Question 1

What are deadenylases?

Answer 1

The slow shortening of the poly(A) tail by adenylate-specific exonucleases called deadenylases

Question 2

What is the general life time of an mRNA?

Answer 2

The time required for deadenylation defines. The lifetime of mammalian mRNAs varies from <10 min to > 9 hours

Question 3

Why is the mRNA degraded after the poly(A) tail has been shortened to ~10 residues?

Answer 3

Loss of PABP binding causes loss of interaction between the 5’ and 3’ ends of the mRNA, decreasing the efficiency of translation and making the mRNA susceptible to degradation

Question 4

What is the correlation between mRNA stability and translational efficiency?

Answer 4

mRNA stability and translational efficiency are inversely correlated

Question 5

How are deadenylated transcripts degraded?

Answer 5

There are 2 ways;

• The m7G cap structure is removed by the heterodimeric Dcp1/Dcp2 decapping enzyme. The decapping enzyme hydrolyses the 5’-5’ triphosphate linkage, releasing m7GDP and generating an RNA with a 5’ monophosphate group. This is a good substrate for 5’->3’ exonucleases
• deadenylated transcripts can be degraded by 3’->5’ exoribonucleases. Exonucleolytic degradation by Xrn1 or the exosome is rapid.
• -The steps of deadenylation or decapping are typically rate-limiting for mRNA turnover

Question 6

Compare and contrast the expression and production levels of stable and unstable mRNAs?

Answer 6

Stable mRNAs will therefore be expressed at higher levels than unstable mRNAs if under the same transcriptional control. However, when the transcriptional rate is changed, an unstable mRNA will achieve its new steady state level far more rapidly than a stable mRNA. Similarly, changes in mRNA stability could cause a rapid change in steady state levels if the transcriptional rate remains constant

Question 7

What is a common feature among unstable mRNAs?

Answer 7

Unstable mRNAs often contain (A/U- rich elements or AREs) within their 3’ UTRs. Typical AREs contain multiple AUUUA motifs within a U-rich region

Question 8

How is ARE-mediated decay regulated?

Answer 8

regulated positively and negatively by the relative expression of ARE binding proteins (AUF1 and HuR, respectively). AUF1 and HuR are regulated by alternative splicing and post- translational modification

Question 9

What are miRNAs?

Answer 9

Short microRNAs (miRNAs) (~ 22 nucleotide long) are generated from larger endogenous precursor RNAs and are found throughout metazoans

Question 10

How are miRNAs processed

Answer 10

Canonical miRNAs are processed from primary transcripts (pri-miRNAs) by Drosha in the nucleus and by Dicer in the cytoplasm

Question 11

What do miRNAs do?

Answer 11

anneal to sites within target mRNAs, typically in the 3’ UTR. RISC complexes can cleave the target mRNAs, recruit deadenylases or repress translation

Question 12

What happens to early stop codons generated from nonsense mutations?

Answer 12

Transcripts with early stop codons are recognised and rapidly degraded by the nonsense- mediated decay (NMD) pathway. NMD is dependent upon the mRNA being actively translated. The context of the translation termination event is sensed through interactions between the ribosome at the termination codon and the structure of the mRNP particle over the 3’ UTR. If termination occurs in an inappropriate context, the ribosome may not interact with the 3’ mRNP domain and then the transcript is targeted to NMD