Fig. (2).

Suggested model for the regulation of miRNA expression. MicroRNA expression may be regulated at both transcriptional and post-transcriptional levels. Step i: transcription activation or repression by DNA and/or histone modification, for binding by transcription factors (TFs) for either activating or repressing the promoter elements of the microRNA gene clusters; Step ii: transcription of miRNA gene by RNA polymerase II (RNAP II) and/or RNA polymerase III (RNAP III); Step iii: putative factors regulating RNAP functions for initiation, elongation, and termination steps of microRNA gene clusters; Step iv: export of the primary transcripts of miRNA (pri-miRNA), followed by further processing to become precursor microRNA (pre-miRNA), which are exported from the nucleus to the cytoplasm; Step v: in the cytoplasm, pre-miRNAs are processed further to mature miRNAs, which can then silence the target message’s expression.