CSB349H1 Lecture Notes - Lecture 12: Piwi, P Element, Transposable Element
14 May 2018
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Lecture 12(a): piRNAs & LncRNAs
PIWI Interacting RNA (piRNA):
• piRNA are class of small RNAs that are exclusively expressed in germline and function in
transposon silencing and epigenetic regulation
o miRNA = 21-25 nucleotides | piRNA = 26-32 nucleotides
• piRNA will form a complex with PIWI proteins; PIWI proteins are in the same family as
the AGO proteins
o piRNA come from long single-stranded precursors (piRNA clusters); these
clusters are not processed by Drosha and Dicer
ð piRNA and miRNA have similar mode of action – however their processing and origin differ
o piRNA => single-stranded precursors | miRNA => double-stranded precursors
ð It is important to keep transposons ‘in check’ within the germ-line because if any mutations
occur it will be found in future offspring
o As seen, during germ-line development a genome-wide demethylation occurs and
transposons are capable of being activated
piRNA Transposon Control:
• piRNA cluster will be transcribed within the nucleus
o piRNA cluster is processed via nucleases and interact with PIWI proteins to form a
piRISC complex within the cytosol
§ piRISC complex has a single-strand guide RNA (anti-sense) that will bind to
complementary transposon sequences and function to degrade them also
can
function to move back into the nucleus and induce heterochromatin formation
• Transcriptional and post-transcriptional silencing
ð Ping-Pong Cycle:
o Starting with one piRISC – will function to cleave target; after cleavage we are left with
an RNA molecule (sense strand) which is capable of binding to another PIWI protein
§ Transposons are found in multiple copies and can be transcribed in the
forward direction
or
reverse direction and give rise to dsRNA
• Molecules that are not dsRNA can be targeted by the two different
PIWI proteins; antisense & sense strand
piRNA Molecular Memory:
• Hybrid dysgenesis: refers to high rate of mutations in the germ line cells of Drosophila
o Results from a cross of males with P element transposon and females without P element
(effect differs depending on which parent has the P element transposon)
a) Male carries the P element- progeny will be sterile; transposons are active and cause mutations
b) Female carries the P element- progeny will be fertile; transposons are inhibited by piRNA
o Mother will transmit piRNA through the egg cell: trans-generation memory through
the transmission of RNA molecules
§ piRNA are present in zygote and function to prevent transposon mutations
