Lariat debranching by RNA DEBRANCHING ENZYME 1 depends on SICKLE in Arabidopsis thaliana

Spliceosome mediated intron removal from precursor mRNAs (pre-mRNAs) generates circular RNAs called intron lariats. RNA DEBRANCHING ENZYME 1 (DBR1) ribonucleases linearize, or debranch, intron lariats to allow their degradation. DBR1 genes occur across eukaryotes and are essential in animals and plants. High levels of intron lariats in the weak Arabidopsis thaliana dbr1-2 allele inhibits primary microRNA (pri-miRNA) processing, disrupting miRNA production and miRNA-regulated growth and development. Arabidopsis sickle (sic) mutants alter pri-miRNA processing and pre-mRNA splicing. This study demonstrates sic mutants accumulate intron lariats matching those in weak dbr alleles. The strong sic-1 and weak dbr1-3 alleles together cause synthetic lethality, while weak sic-3 with dbr1-3 has intron lariat accumulation like sic-3. Further, sic-3, dbr1-3, and sic-3 dbr1-3 similarly alter circadian rhythms and growth of roots and rosettes. The conserved MPLKIP amino acid motif in SIC mediates physical interaction with DBR1 in vitro and is required for intron lariat debranching in vivo. Thus, MPLKIP containing proteins, like SIC and human TTDN1, act with cognate DBR1 proteins to maintain RNA homeostasis critical for growth and development.