Atta ur Rahman school of biological Science National University of Sciences & Technology CMB-234:Molecular Biology

RNA interference (RNAi) is a biological process in which RNA molecules inhibit gene expression, typically by causing the destruction of specific mRNA molecules In 2006, Andrew Fire and Craig C. Mello shared the Nobel Prize in Physiology or Medicine for their work on RNA interference in the nematode worm C. elegans, which they published in 1998

Interference can be at Transcriptional gene silencing (TGS) Post transcriptional gene silencing (PTGS) Co - transcriptional gene silencing (CTGS) Chromatin dependent gene silencing (CDGS)

Three types include Micro RNA (21 – 25nt) Small interfering RNA (21 – 25nt) PIWI interacting RNA (24 – 31nt)

Dicer Convergent promoters 5’ 3’ Aberrant RNA RDRP Dicer TRAMP Degradation of RNA by forming exosome

DROSHADICER

AAAAAA PIWI AUB or AGO- 3

NUCLEUS CELL nt long Argonaut : - PIWI - PAZ - MID RISC Dicer = a RNase - III This process can take place here as well RISC = RITS

NUCLEUS CELL Argonaut : - PIWI - PAZ - MID RISC Drosha & Dicer ssRNA ~ nt Translational inhibition May be present in here

NUCLEUS CELL Mechanism of action of pi RNA is not yet fully elucidated but it is known that it involves the slicer activity of PIWI Clade It is responsible for controlling the movement of transposons in the genome

 Gene regulation  RNA sequences (siRNA and miRNA) that are complementary to parts of a promoter can increase gene transcription, a phenomenon dubbed RNA activation.  Part of the mechanism for how these RNA upregulate genes is known: dicer and argonaute are involved, possibly via histone demethylation.  miRNAs have also been proposed to upregulate their target genes upon cell cycle arrest, although the underlying mechanisms have not been elucidated.

HETEROCHROMATIN Repetitive DNA sequences & Transposons Important role in chromosome transmission Maintenance of genome stability Regulation of gene expression Concentrated in centromeres in all except yeast Also in telomeres & genome loci Arrangement 1. Protein & RNA - Sequence 2. Changes in chromatin structure x H3K9 methylation done by Chromodomain-1(Chp1) x loss of Chp-1, decrease in methylation x purification of Chp-1 has shown its association with Ago1 in RITS x This means that RNAi may have some role in heterochromatin silencing as it activates the same proteins that are activated by other cellular factors for this control

 Gene Down regulation ▪ Endogenously expressed miRNAs, are most important in translational repression and in the regulation of development, especially on the timing of morphogenesis and the maintenance of undifferentiated or incompletely differentiated cell types such as stem cells

 In Fission yeast & some ciliates, loss of heterochromatin gene silencing by del in  Argonaute (Ago1)  Dicer (dcr1)  RNA dependent RNA polymerase (rdp1)  In Arabidopsis thaliana, RNA directed DNA methylation (H3K9)requires 2 proteins  Dicer (DLC-3)  Argonaute (AGO-4)  In Drosophila melanogaster, repeat induced gene silencing by  Polycomb group of genes (PcG)  PIWI  DCR-1 RNAi IS REQUIRED FOR SOME BUT NOT ALL CHROMATIN MEDIATED SILENCING EVENTS

 RNA interference is a vital part of the immune response to  Viruses and  Other foreign genetic material, especially in plants where it may also prevent the self- propagation of transposons.

Most mechanisms still unknown In future we can employ this mechanism to up or down regulate a gene, or to fight viral infections Future Prospects

Small RNAs in transcriptional gene silencing and genome defence Danesh Moazed Nature 457, Reference: