RNA Interference

co-suppression

early finding in RNA interference

a series of observations involving some sort of homology-dependent gene silencing collectively called this

molecular basis was not clear

RNA interference

process of mRNA degradation that is induced by double-stranded RNA in a sequence-specific manner

triggered by dsRNA

results in specific, post-transcriptional silencing of genes of similar sequence

involves action of protein complexes including ribonucleases, RNA-binding proteins, polymerases, etc.

amplification steps reinforce the effect

in some organisms is part of a broader set of cellular responses to RNA that play roles in regulation of translation, transcription, chromatin structure, and genome integrity

basic RNAi pathway

initiation and effector steps

Dicer binds dsRNA and cleaves into ~22 nt fragments →siRNAs (small interfering RNAs)

RISC (RNA induced silencing complex) binds one siRNA

RISC activation (siRNA unwinding, ATP dependent)

activated RISC binds a target mRNA and degrades it

initiation step- RNAi

Dicer generates precisely sized fragments with 2-nuc overhands, phosphorylated at 5’ (siRNAs)

Dicer

a member of RNase III family of double-strand-specific endonucleases

effector step- RNAi

unwound fragment directs the particle to a cognate mRNA by WC base pairing

gene expression silenced by mRNA degradation and/or translation inhibition

RISC

RNA induced silencing complex

includes Argonaute (another endoribonuclease)

recognizes Dicer-generated dsRNA fragments

amplification- RNAi

some siRNA unwind without being associated with RISC

the antisense strand can bind to its complementary region of the corresponding mRNA, serving as a primer for a RdRP which will generate new dsRNA of the target gene

RdRP generated dsRNA substrate for Dicer, and more siRNA made

in plants, sense siRNA can be copied by an RdRP that does not require priming, resulting in new siRNA

siRNA can go from cell to cell, and so silencing can propagate to other tissues

miRNAs (micro interfering RNA)

encode small RNAs that regulate gene expression through post-transcriptional repression

nascent (these) transcripts are processed sequentially by two RNase III enzymes, Microprocessor and Dicer, to yield mature (these) duplexes, ranging from 18bp to 24bp in length

fragments associate with RISC and depending on complementarity lead to degradation of mRNA or translational repression

also control expansion of mobile genetic elements: their genes express dsRNA with similarity to transposons, which prevents their proliferation by silencing their mRNAs

piRNA

a type of miRNA specific to animals, expressed in germline tissues

targets certain transposons from proliferating

miRNA transcription in normal tissues

processing and binding to complementary sequences on target mRNA results in repression of target-gene expression through a block in protein translation or altered mRNA stability

overall result is normal rates of cellular growth, proliferation, differentiation, and cell death

reduction or deletion of miRNA

in case of one that functions as a tumour suppressor, leads to tumour formation

can occur because of defects at any stage of (this) biogenesis and ultimately leads to inappropriate expression of the (this)-target oncoprotein

overall outcome might involve increased proliferation, invasiveness or angiogenesis, decreased levels of apoptosis, or undifferentiated or de-differentiated tissue, ultimately leading to tumour formation

amplification or overexpression of miRNA

for those in oncogenic role, results in tumour formation

in this situation, increased amounts of this which might be produced at inappropriate times or in the wrong tissues, would eliminate the expression of (this)-target tumour-suppressor gene and lead to cancer progression

increased levels of mature this might occur because of amplification of this gene, a constitutively active promoter, increased efficiency in this processing, or increased stability of this