The Arabidopsis NOT4A E3 ligase promotes PGR3 expression and regulates chloroplast translation (Nature Comms)

Pentatricopeptide repeat (PPR) domain containing proteins are nuclear encoded with functions in the chloroplast, but regulation of PPR gene expression in the nucleus before import to the chloroplast has not been well studied. Bailey et al. identified and characterized the ubiquitin ligase NOT4A and its regulatory function on PGR3, a PPR protein important for several photosynthetic processes. A not4a mutant displayed pale yellow coloration, delayed flowering, and starch depletion compared to wild-type. RNA sequencing and proteomics confirmed that several photosystem II and cytochrome b₆f complex proteins are mis-regulated in the not4a mutant. Chloroplast 30 S ribosome subunit proteins were depleted in not4a although they were elevated in the RNA dataset. The not4a mutant was hypersensitive to the  plastid ribosome inhibitor lincomycin, and wild-type plants treated with lincomycin phenocopied untreated not4a, suggesting that chloroplast mRNA translation is impaired in not4a mutant. Lastly, PGR3 expression was significantly downregulated in not4a, and PGR3 expression complements impaired chloroplast functions not4a mutant. Taken together, the research shows that NOT4A is essential for proper photosynthetic function and positively regulates PGR3 and chloroplast translation. (Summary by Toluwase Olukayode @toluxylic) Nature Comms. 10.1038/s41467-020-20506-4