Dual Role of a SAS10/C1D Family Protein in Ribosomal RNA Gene Expression and Processing Is Essential for Reproduction in Arabidopsis thaliana

摘要

In eukaryotic cells, ribosomal RNAs (rRNAs) are transcribed, processed, and assembled with ribosomal proteins in the nucleolus. Regulatory mechanisms of rRNA gene (rDNA) transcription and processing remain elusive in plants, especially their connection to nucleolar organization. We performed an in silico screen for essential genes of unknown function in Arabidopsis thaliana and identified Thallo ( THAL ) encoding a SAS10/C1D family protein. THAL disruption caused enlarged nucleoli in arrested embryos, aberrant processing of precursor rRNAs at the 5’ External Transcribed Spacer, and repression of the major rDNA variant ( VAR1 ). THAL overexpression lines showed de-repression of VAR1 and overall reversed effects on rRNA processing sites. Strikingly, THAL overexpression also induced formation of multiple nucleoli per nucleus phenotypic of mutants of heterochromatin factors. THAL physically associated with histone chaperone Nucleolin 1 (NUC1), histone-binding NUC2, and histone demethylase Jumonji 14 (JMJ14) in bimolecular fluorescence complementation assay, suggesting that it participates in chromatin regulation. Furthermore, investigation of truncated THAL proteins revealed that the SAS10 C-terminal domain is likely important for its function in chromatin configuration. THAL also interacted with putative Small Subunit processome components, including previously unreported Arabidopsis homologue of yeast M Phase Phosphoprotein 10 (MPP10). Our results uncovering the dual role of THAL in transcription and processing events critical for proper rRNA biogenesis and nucleolar organization during reproduction are the first to define the function of SAS10/C1D family members in plants. Author Summary The rRNA regulatory network underlying the structure and function of the plant nucleolus is largely unknown. We identified a previously uncharacterized SAS10/C1D family protein THAL as a novel component important for both rRNA gene expression and processing, which further impacts nucleolar structure. thal-2 mutant had enlarged nucleoli whereas THAL overexpressed cells displayed multiple nucleoli resembling heterochromatin decondensation. THAL associated with the well-known histone demethylase JMJ14, which implies not only a role of THAL in chromatin regulation but also a role of JMJ14 in rDNA transcription. In addition, THAL interacted with putative Small Subunit (SSU) processome components including nucleolin and previously unreported Arabidopsis homologue of yeast MPP10, thus our work provides initial evidence of the existence and partial constituents of the putative SSU processome in Arabidopsis. Our study adds a piece to the rRNA network puzzle, provides insight into SAS10/C1D family function and prompts investigation of THAL mammalian homologues, especially since nucleolar dysfunction is characteristic of many diseases including cancer.