SMALL ORGAN 4 is a ribosome biogenesis factor involved in 5.8S rRNA maturation

Ribosome biogenesis is crucial for cellular metabolism and has important implications for disease and aging. Human glioma tumor-suppressor candidate region gene 2 (GLTSCR2) and yeast (Saccharomyces cerevisiae) Nucleolar protein 53 (Nop53) are orthologous proteins with demonstrated roles as ribosome biogenesis factors (RBFs); knockdown of GLTSCR2 impairs maturation of 18S and 5.8S ribosomal RNAs (rRNAs), and Nop53 is required for maturation of 5.8S and 25S rRNAs. Here, we characterized SMALL ORGAN 4 (SMO4), the most likely ortholog of human GLTSCR2 and yeast Nop53 in Arabidopsis (Arabidopsis thaliana). Loss of function of SMO4 results in a mild morphological phenotype; however, we found that smo4 mutants exhibit strong cytological and molecular phenotypes: nucleolar hypertrophy and disorganization, overaccumulation of 5.8S and 18S rRNA precursors, and an imbalanced 40S:60S ribosome subunit ratio. Like yeast Nop53 and human GLTSCR2, Arabidopsis SMO4 participates in 5.8S rRNA maturation. In yeast, Nop53 cooperates with mRNA transport 4 (Mtr4) for 5.8S rRNA maturation. In Arabidopsis, we found that SMO4 plays similar roles in the 5.8S rRNA maturation pathway than those described for MTR4. However, SMO4 seems not participate in the degradation of by-products derived from the 5'-ETS of 45S pre-rRNA, as MTR4 does.