SMALL ORGAN4 Is a Ribosome Biogenesis Factor Involved in 5.8S Ribosomal RNA Maturation

Area of Science:

• Molecular Biology
• Cell Biology
• Genetics

Background:

• Ribosome biogenesis is fundamental for cellular function, impacting metabolism, disease, and aging.
• Human GLTSCR2 and yeast Nop53 are known ribosome biogenesis factors with critical roles in rRNA maturation.
• Understanding orthologous proteins in plants like Arabidopsis thaliana can reveal conserved biological mechanisms.

Purpose of the Study:

• To characterize the function of SMALL ORGAN4 (SMO4), the putative ortholog of human GLTSCR2 and yeast Nop53, in Arabidopsis thaliana.
• To investigate the role of SMO4 in ribosome biogenesis and rRNA maturation in plants.
• To compare the function of SMO4 with its orthologs in yeast and humans.

Main Methods:

• Genetic analysis of Arabidopsis thaliana SMO4 loss-of-function mutants.
• Cytological examination of nucleolar structure in wild-type and mutant plants.
• Molecular analysis of rRNA precursor accumulation and ribosome subunit ratios.

Main Results:

• SMO4 loss-of-function mutants exhibit nucleolar hypertrophy and disorganization.
• Mutants show overaccumulation of 5.8S and 18S rRNA precursors, indicating impaired maturation.
• An imbalanced 40S:60S ribosome subunit ratio was observed in smo4 mutants.
• SMO4 is confirmed to participate in 5.8S rRNA maturation, similar to its yeast and human orthologs.
• SMO4's role in 5.8S rRNA maturation parallels that of MTR4 in Arabidopsis, but it does not appear to be involved in 45S pre-rRNA 5'-ETS byproduct degradation.

Conclusions:

• Arabidopsis SMO4 is a crucial factor in ribosome biogenesis, specifically in 5.8S rRNA maturation.
• SMO4 plays a conserved role in ribosome production, analogous to its orthologs in yeast and humans.
• While SMO4 shares functional similarities with MTR4 in rRNA processing, it has distinct roles in byproduct degradation pathways.