small activating RNA resource for gene upregulation
RNA activation (RNAa) is a recently discovered biological phenomenon associated with targeting the promoter region of gene, its activation and expression by small activating RNAs (saRNA). The process of RNAa and RNA interference (RNAi) is provided in the figure below and each step discussed.
Hypothesised RNAa mechanism and comparision to RNAi
Figure legends:
Diagrammatic representation of the RNA activation (RNAa) using small activating RNA (saRNA) and its comparison with the RNA interference (RNAi). Steps are explained by the alphabetical order with letter and arrow directions. Single arrow represents RNAa while double arrows represents (RNAi). Upper panel (delivery methods) reveals the different delivery methods of the saRNAs as well as siRNAs (both are 19 to 21 nucleotide double stranded RNA molecules).
(A) Starting from left most is naked saRNA, viral delivery, plasmid based delivery, lipid based delivery and microinjection (next is the pictorial representation of siRNA).
(B-C) saRNA enters into the cell cytoplasm and binds to argonaute (AGO2) proteins followed by binding of other proteins and forming the RNA inducing transcriptional activation(RITA) complex. The antisense is retained and sense strand is discarded and degraded as in siRNA. RITA complex has many proteins associated which are involved in different cellular processed like chromatin and dsRNA binding proteins; transcription as well as translation associated proteins; metabolic, cytoskeleton proteins and ribo-nulcear proteins
(D-E) The RITA enters into the nucleus possibly by two ways one is active transport via the nuclear pore complex (NPC) or passively when the nuclear membrane is disintegrated during cell division. Inside nucleus RNAa is shown to act by two mechanisms.
(F-H) RITA epigenetically changes the heterochromatin of the cell and opens new gene locations based on the recognition of the saRNA antisense. RITA also found to be associated with RNA polymerase (RNAP) thus enhances in the nearby gene expression.
(I) The transcription occurs for target gene and mRNAs are formed.
(J-K) other mechanism of RNAa is that it binds to and releases the transcriptional initiation complex where RNA polymerase complex (RNAP-C) is bound to euchromatin but cannot initiate the gene expression because of the its halting by other non coding RNA or some structural conformations associated with the promoter.
(L) The Pre-imitation complex is released and the mRNA expressions starts form the Transcriptional start site (TSS).
(N) The mRNAs are processed and are re-localized to cytoplasm for translation.
(O-Q) siRNA inside the cell binds with AGO and associated proteins and forms the RISC (RNA induced RNA silencing) complex (double arrow pathway). The RISC complex binds the antisense (guide) strand and discards the passenger strand.
(R-T) the RISC bind to the cognate mRNA leading to its degradation and hence leading to the post transcriptional gene silencing.
