Formation of Circular RNAs.

Circular RNAs (circRNAs) constitute a class of covalently closed, single-stranded RNA molecules that form a continuous loop structure, setting them apart from linear RNA molecules. These circular entities arise through a distinct biogenetic process termed "backsplicing," which involves the covalent bonding of the 5' site of an upstream exon with the 3' site of the same or a downstream exon during pre-messenger RNA (pre-mRNA) splicing. Such a direct backsplicing model exists along with the exon- and lariat-skipping models of circRNA generation, giving rise to three major types of circRNAs, i.e., exonic (EcircRNAs), intronic (IcircRNAs), and exonic-intronic circRNAs (EIcircRNAs). A complex process of interaction between cis-acting elements and trans-acting factors governs circRNA synthesis. In particular, reverse complementary matches (RCMs) and RNA-binding proteins (RBPs) play critical roles in circRNA formation. Complementary sequences increase base pairing, which aids in the alignment of splice sites for backsplicing. RBPs can either accelerate or inhibit circRNA synthesis by binding to certain sequences or structures, thereby altering backsplicing efficiency. The careful balance of cis-acting components and trans-acting factors highlights the complex regulatory mechanisms driving circRNA synthesis, which contributes to the variety and functional relevance of these transcripts in cellular activities. In this chapter, we review the variables that influence circRNA biogenesis and highlight the origins of different types of these transcripts in humans.