5'UTR Editing of the ribosomal protein UL3Z gene unveils its critical roles in pre-rRNA processing and global mRNA translation dynamics

The EMBRYO DEFECTIVE 2207 (EMB2207) gene, encoding ribosomal protein UL3Z, is critical for embryonic development in Arabidopsis, with loss of function resulting in embryo lethality. Despite its importance, the role of UL3Z in the complicated protein translation machinery in plants remains poorly understood due to the lack of viable hypomorphic alleles. In this study, we utilized CRISPR/Cas9 to edit the 5' untranslated region (5'UTR) of UL3Z, generating 5 ul3z mutants with varying degrees of reduced expression levels of UL3Z proteins. The mutant with the lowest expression exhibited the most severe developmental defects. In contrast, null mutants of its paralog UL3Y displayed no observable phenotypes. Interestingly, expression of UL3Y driven by the UL3Z/EMB2207 promoter successfully rescued the phenotypes of ul3z, demonstrating that these 2 paralogous ribosomal proteins actually possess functionally interchangeable roles. GUS staining results showed that UL3Z was constitutively expressed in all examined tissues, while UL3Y was only appreciably expressed in specific tissues. Molecular analysis further revealed the accumulation of ribosomal RNA (rRNA) maturation intermediates and increased polysome levels in ul3z mutants, indicating compromised pre-rRNA processing and disturbed global mRNA translation. Interestingly, 3' ends of many rRNA precursors in ul3z had higher frequency of non-encoded tails compared with Col-0. This study demonstrates that CRISPR/Cas9-mediated 5'UTR editing is an effective tool for generating viable hypomorphic alleles of lethal genes and uncovers the critical roles of UL3Z/EMB2207 in pre-rRNA processing and the maintenance of appropriate mRNA translation on ribosomes, underscoring its importance in plant development.