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. 2021 Sep 16;8:714958. doi: 10.3389/fmed.2021.714958

Figure 1.

Figure 1

The biogenesis, regulation, and functions of circRNAs. CircRNAs are transcribed from pre-mRNAs through the non-canonical back-splicing event. The back-splicing process involves different mechanisms, including circularization by base paring of intronic sequences or RBPs dimerization, exon skipping, and intronic lariat, which generates three types of circRNAs: ecircRNAs, ciRNAs, and EIciRNAs. Some molecules, including QKI, Mbl, NF90/NF110, positively regulate the biogenesis of circRNAs, while ADAR1 and DHX9 inhibit circRNAs formation. CircRNA biogenesis per se competes with canonical linear splicing. The most well-studied functions of circRNAs is to act as miRNA sponges to modulate the expression of miRNA target genes. CircRNAs can bind to RNA pol II or other RBPs to affect transcription levels of their parental genes, especially for nuclear-retained ciRNAs and EIciRNAs. CircRNAs can act as sponges, decoys, scaffolds for proteins or regulate protein trafficking through interacting with specific proteins. Certain circRNAs can be translated into proteins under stress circumstances.