Biogenesis of circular RNAs and from the / gene.

( ) encodes a fly homolog of mammalian that is vital for early fly development as well as for optic lobe development. Interestingly, was reported to produce a circular RNA (circRNA). In this study, we identified 12 different circRNAs, which are either mono- or multi-exonic forms. The most abundant forms are circE2 carrying the second exon only and bi-exonic circE3-E4. Levels of circE2 show an age-dependent increase in adult heads, supporting a general trend of high accumulation of circRNAs in aged fly brains. Aligning sequences of introns flanking exons uncovered two pairs of intronic complementary sequences (ICSs); one pair residing in introns 1 and 2 and the other in introns 2 and 4. The first pair was demonstrated to be essential for the circE2 production in cell-based assays; furthermore, deletion of the region including potential ICS components in the intron-2 reduced production of circE2 and circE3-E4 by 80%, indicating them to be essential for the biogenesis of these isoforms. Besides the ICS, the intron regions immediately abutting exons seemed to be responsible for a basal level of circRNA formation. Moreover, the replacement of -ICS with those derived from was comparably effective in -circRNA production, buttressing the importance of the hairpin-loop structure formed by ICS for the biogenesis of circRNA. Lastly, overexpressed affected outcomes of both linear and circular RNAs from the endogenous locus, suggesting that Scro plays a direct or indirect role in regulating expression levels of either or both forms.