Dynamics of miRNA biogenesis and nuclear transport.

MicroRNAs (miRNAs) are short noncoding RNA sequences ~22 nucleotides in length that play an important role in gene regulation-transcription and translation. The processing of these miRNAs takes place in both the nucleus and the cytoplasm while the final maturation occurs in the cytoplasm. Some mature miRNAs with nuclear localisation signals (NLS) are transported back to the nucleus and some remain in the cytoplasm. The functional roles of these miRNAs are seen in both the nucleus and the cytoplasm. In the nucleus, miRNAs regulate gene expression by binding to the targeted promoter sequences and affect either the transcriptional gene silencing (TGS) or transcriptional gene activation (TGA). In the cytoplasm, targeted mRNAs are translationally repressed or cleaved based on the complementarity between the two sequences at the seed region of miRNA and mRNA. The selective transport of mature miRNAs to the nucleus follows the classical nuclear import mechanism. The classical nuclear import mechanism is a highly regulated process, involving exportins and importins. The nuclear pore complex (NPC) regulates all these transport events like a gate keeper. The half-life of miRNAs is rather low, so within a short time miRNAs perform their function. Temporal studies of miRNA biogenesis are, therefore, useful. We have carried out simulation studies for important miRNA biogenesis steps and also classical nuclear import mechanism using ordinary differential equation (ODE) solver in the Octave software.