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The checkpoint kinase TOR (target of rapamycin) regulates expression of a nuclear‐encoded chloroplast RelA‐SpoT homolog ( RSH ) and modulates chloroplast ribosomal RNA synthesis in a unicellular red alga
Author(s) -
Imamura Sousuke,
Nomura Yuhta,
Takemura Tokiaki,
Pancha Imran,
Taki Keiko,
Toguchi Kazuki,
Tozawa Yuzuru,
Tanaka Kan
Publication year - 2018
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1111/tpj.13859
Subject(s) - chloroplast , microbiology and biotechnology , biology , rna , ribosomal rna , gene , genetics
Summary Chloroplasts are plant organelles that carry out oxygenic photosynthesis. Chloroplast biogenesis depends upon chloroplast ribosomes and their translational activity. However, regulation of chloroplast ribosome biogenesis remains an important unanswered question. In this study, we found that inhibition of target of rapamycin ( TOR ), a general eukaryotic checkpoint kinase, results in a decline in chloroplast ribosomal RNA ( rRNA ) transcription in the unicellular red alga, Cyanidioschyzon merolae . Upon TOR inhibition, transcriptomics and other analyses revealed increased expression of a nuclear‐encoded chloroplast RelA‐SpoT homolog ( RSH ) gene ( Cm RSH 4b ), which encodes a homolog of the guanosine 3′‐diphosphate 5′‐diphosphate (ppGpp) synthetases that modulate rRNA synthesis in bacteria. Using an Escherichia coli mutant lacking ppGpp, Cm RSH 4b was demonstrated to have ppGpp synthetase activity. Expression analysis of a green fluorescent protein‐fused protein indicated that Cm RSH 4b localizes to the chloroplast, and overexpression of the Cm RSH 4b gene resulted in a decrease of chloroplast rRNA synthesis concomitant with growth inhibition and reduction of chloroplast size. Biochemical analyses using C. merolae cell lysates or purified recombinant proteins revealed that ppGpp inhibits bacteria‐type RNA polymerase‐dependent chloroplast rRNA synthesis as well as a chloroplast guanylate kinase. These results suggest that Cm RSH 4b‐dependent ppGpp synthesis in chloroplasts is an important regulator of chloroplast rRNA transcription. Nuclear and mitochondrial rRNA transcription were both reduced by TOR inhibition, suggesting that the biogeneses of the three independent ribosome systems are interconnected by TOR in plant cells.