A New Type of Circular RNA derived from Nonconventional Introns in Nuclear Genes of Euglenids.

• Nuclear, protein-coding genes of euglenids contain atypical, nonconventional introns. • Released nonconventional introns of euglenids adopt full-length circular form. • Here, we describe a new type of intronic circular RNA in Eukarya. • Circular form of released nonconventional introns may be involved in intron mobility. Nuclear protein-coding genes of euglenids (Discoba, Euglenozoa, Euglenida) contain conventional (spliceosomal) and nonconventional introns. The latter have been found only in euglenozoans. A unique feature of nonconventional introns is the ability to form a stable and slightly conserved RNA secondary structure bringing together intron ends and placing adjacent exons in proximity. To date, little is known about the mechanism of their excision (e.g. whether it involves the spliceosome or not). The tubA gene of Euglena gracilis harbors three conventional and three nonconventional introns. While the conventional introns are excised as lariats, nonconventional introns are present in the cell solely as circular RNAs with full-length ends. Based on this discovery as well as on previous observations indicating that nonconventional introns are observed frequently at unique positions of genes, we suggest that this new type of intronic circRNA might play a role in intron mobility. [ABSTRACT FROM AUTHOR]