Question

What is microRNA. Provide two examples from literature how microRNA regulates mRNA.

Answer 1

microRNAs are small non coding RNAs that are19-24 nucleotides in length. These RNAs regulate the gene expression at the post transcriptional level by binding to the 3'-UTR of mRNAs.

Example 1: Let-7microRNAmediated mRNA deadynylation and translational repression in a mammalian cell free system.

microRNAs are incorporated intomiRNP complexes and regulate protein expression post transcriptionally through binding to 3'-untargeted regions of mRNAs.Here we describe the recapitulation of Let-7 miRNA mediated translational repression in cell free system. Which was established with extracts prepared from HEK293F cells over expressing miRNA pathway components.In this system both cap and poly A tail are required for the translational repression and let-7 directs the deadynylation of target mRNAs. Which suggests let-7 miRNPs containing Argonaute and GW-182 impair the synergistic enhancement of translation by the 5'-cap and 3'-poly tail resulting the translational repression.

Example2: microRNAs are approximately 21 nucleotide long RNA molecules regulating gene expression in multicellular eukaryotes. In metazoa miRNAs act by imperfectly base pairing with the 3' untranslated region of target messenger RNAs and repressing protein accumulation by an unknown mechanism. It demonsrademo that let-7 microribonucloproteins or the tethering of Argonaute proteins to reporter mRNAs in human cells inhibit translation initiation. M(7) -G-cap independent translation is not subject to repression. Suggesting that miRNPs interfere with recognition of cap. Repressed mRNAs, Ago proteins and miRNAs were all found to accumulate in processing bodies. It is proposed that localisation of mRNAs to these structures is a consequence of translational repression.