The homologues of Arabidopsis JANUS in yeast and humans are splicing factor 3B4 (SF3B4) and human sap homologue 49 (HSH49), respectively. Both bind to the U2 small nuclear RNA and participate in the assembly of the U2 small nuclear ribonucleoprotein, a component of the spliceosome. However, in Arabidopsis, JANUS was previously found to recruit RNA polymerase II to WUSCHEL RELATED HOMEOBOX 2 (WOX2), PIN-FORMED 7 (PIN7) and PLETHORA 1 (PLT1) for proper transcription, but was not seen to be involved in the splicing of these targets. Consequently, janus mutants are embryo lethal owing to misexpression of WOX2 and PIN7 and abnormal embryonic cell divisions. As JANUS is a member of a network of factors involved in miRNA biogenesis, the authors of this study investigated whether JANUS has further functions beyond transcriptional regulation.
The researchers found that JANUS can interact with SERRATE (SE) and DICER LIKE 1 (DCL1), two components of the DCL1 complex involved in pri-miRNA processing. In a hypomorphic janus mutant line, miRNAs are globally reduced and some miRNA targets are increased, suggesting defective gene silencing. As several pri-miRNAs are reduced and RNA polymerase II has a lower occupancy at some miRNA gene promoters, JANUS seems to have a role in the transcriptional regulation of miRNA genes. JANUS was also found to bind pri-miRNA stem-loops and is required for efficient processing. There were significantly fewer dicing bodies in lines with reduced JANUS expression, and assembly of DCL1 complexes was reduced. Finally, RNA-sequencing analysis showed that the intron retention rate was higher in lines with reduced JANUS expression. These data demonstrate that while JANUS does play a role in splicing, this role seems to be independent of its function in miRNA biogenesis or translational regulation.
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