Micro RNA are single -stranded small RNA molecules of about 19-25 nucleotides in length, encoded in genomes of plants and animals. These are non coding RNA. These regulate gene expressions by targeting one or more mRNA s for translation repression or cleavage (degradation). 

Introduction

Micro RNA are single-stranded, non encoding, small regulatory RNA molecules of about 19-25 nucleotide in length, expressed in a variety of different cell types from several animals species, ranging from Caenorhabditis elegans to humans, as well as in plants. First time miRNA was reported by LEE, Feinbaum & Ambros (1993) in Caenorhabditis elegans called as lin-4 which was repressor of lin-14, which encodes for a nuclear protein; 7 years later second miRNA was reported in C. elegans which was called let-7. Both were found to down regulate particular mRNA. Since the discovery of let-7, thousand of miRNA has been identified in many organisms. 

Bio genesis or Formation & Processing

Transcription (pri-miRNA)

• miRNA are initially transcripted by non coding part of DNA.
• These gene encoding miRNA are much longer than processed mature miRNA.
• These transcripts are referred to as primary miRNA (pri-miRNA).
• Transcribed by Polymerase 2, include 5' caps & 3' poly A tail (Smalheiser 2003, Cai 2004) in the nuclease.
• The miRNA portion of this pri-miRNA likely forms a hairpin with signals for dsRNA-specific nuclease cleavage.
• pri-miRNA can be of upto 1000 nucleotide.

Processing (pri-miRNA to pre-miRNA)

• The double stranded ribonucleaese Dorsha digests the pri-miRNA with the help of signals on pri-miRNA hairpin portion & releases hairpin structure called as precursor miRNA (pre-miRNA) of 60-110 nucleotide. Actually this digestion or processing is done by a protein complex called as Microprocessor complex which consist of Dorsha (a ribonuclease for digestion) & Pasha(a RNA binding protein). 

NOTE:

Dorsha facilitate overall production rate of miRNA by generating the 3' protruding ends, which are recognized efficiently by Exportin-5 & Dicer.

Pasha is Dorsha interacting protein with two double stranded RNA binding Domain. Pasha is also known as "DiGorge syndrome chromosomal region8 (DGCR8)

• Now this hairpin structure i.e. precursor miRNA is brought by Exportin-5 out of the nucleus, in cytoplasm. Exportin recognizes the 2 or 3 nucleotides, 3' overhanging left by Dorsha. Exportin binds directly & specifically to correctly processed pre-miRNA & bring it out of nucleus to cytoplasm.
• In cytoplasm, this pre-miRNA is acted on by RNase III Dicer. Dicer cleaves the pre-miRNA removing the loop structure & resultant structure is called as miRNA duplex. The resulting double stranded RNA has 1-4nucleotide 3' overhanging on the either end. Only one of the two strands is the mature miRNA. Dicer also induces formation of RISC(RNA induced silencing complex).
• At this stage miRNA duplex's one strand binds with RISC, which is Mg+ dependent complex. Which strand will bind depends on stability of 5' end of either strands & is selected with argonaute protein which is complex of RISC. This selected strand is called guide strand. The remaining strand is known as antiguide strand or passenger strand, is degraded as a RISC substrate.

Regulation of gene by miRNA-RISC

The mRNA can be either translationaly repressed(partly) by attaching/binding of miRNA or degraded by binding of miRNA-RISC, depending upon what extent miRNA is complementary to mRNA. Cleavage or degradation by miRNA are mostly reported in plants. In animal translation repression seem to be the primary mode of action of miRNA.

Function

In animals miRNA are mainly seen to cause down regulation of gene & stopping protein production by repressing mRNA. Sometime they have also been reported to initiate protein production by initiating promoter site. In plants their main function seem to degrade the mRNA. 

Several research groups have provided evidence that miRNA act as key regulator of the process as diverse as early development, cell proliferation, cell death, apoptosis, fat metabolism & cell differentiation.

Some studies also shows miRNA role in chronic lymphocytic leukemia (Calin,2004) brain development(Krichevsky,2003),colonic adenocarcinoma(Michael,2003), Burkitt's Lymphoma(Metzler,2004) & viral infection(Pfeffer,2004),suggesting possible link between miRNAs & viral disease, neuro development & cancer.

        Writer: Abhishek Sharma