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

Abstract

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 (CmRSH4b), 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, CmRSH4b was demonstrated to have ppGpp synthetase activity. Expression analysis of a green fluorescent protein-fused protein indicated that CmRSH4b localizes to the chloroplast, and overexpression of the CmRSH4b 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 CmRSH4b-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.

ppGpp influences protein protection, growth and photosynthesis in Phaeodactylum tricornutum

“…However, very little is known about the role of (p)ppGpp in algae, and in particular in algae of the red lineage. A recent study in the extremophile red alga Cyanidioschyzon merolae showed that overexpression of CmRSH4b, a functional (p)ppGpp synthetase, results in a reduction in chloroplast size and decreased chloroplast rRNA transcription, although (p)ppGpp levels were not measured (Imamura et al , 2018). The situation in diatoms, with their mixed red-green heritage and specific lifestyles, is even less clear.…”

StABI5 Involved in the Regulation of Chloroplast Development and Photosynthesis in Potato

“…The ability of photosynthesis determines plant growth status, while chloroplast biogenesis and function affect the efficiency of photosynthesis. Recent studies have shown that photosynthesis is regulated by the target of rapamycin (TOR) and ABA signaling pathways [42,46,47]. TOR signaling acts as a positive regulatory factor to promote plant chloroplast biogenesis and photosynthesis, whereas ABA signaling acts as a negative regulatory signal to inhibit photosynthesis [42,[47][48][49][50].…”

“…Recent studies have shown that photosynthesis is regulated by the target of rapamycin (TOR) and ABA signaling pathways [42,46,47]. TOR signaling acts as a positive regulatory factor to promote plant chloroplast biogenesis and photosynthesis, whereas ABA signaling acts as a negative regulatory signal to inhibit photosynthesis [42,[47][48][49][50]. The transcription factor ABI4 of the ABA signaling pathway binds to the promoter regions of photosynthetic genes such as LHCB and RBCS, suppressing the expression of these genes and thus inhibiting photosynthesis [51,52].…”

Golden magic: RSH enzymes for (p)ppGpp metabolism in the diatom Phaeodactylum tricornutum

“…PtRSH4a and PtRSH4b are also unusual among (p)ppGpp synthetases due to the substitution of a residue corresponding to SAS1 G45 / Rel seq G240. This conserved residue has previously been shown to be necessary for (p)ppGpp synthetase activity (Wendrich & Marahiel, 1997; Hogg et al , 2004) and substitution of this residue is widely used to infer the loss of (p)ppGpp synthetase activity (Masuda et al , 2008; Mizusawa et al , 2008; Sato et al , 2015; Ito et al , 2017; Imamura et al , 2018). Modelling of the active site of PtRSH4a and PtRSH4b shows that the glycine substitution does not appear to affect ATP binding by the neighbouring arginine (Fig.…”

“…Plastid-targeted RSH enzymes from the RSH1, RSH2/3 and RSH4 families have been identified in many green and red algae, yet the metabolism and functions of (p)ppGpp in these organisms have barely been investigated (Atkinson et al , 2011; Ito et al , 2017; Field, 2018). One exception is a recent report on the red alga Cyanidioschyzon merolae , where CmRSH4b, a member of the RSH4 family, was shown to possess (p)ppGpp synthetase activity (Imamura et al , 2018). Interestingly, the inducible expression of CmRSH4b in C. merolae results in a reduction in plastid size and rRNA transcription in a similar manner to the expression of a (p)ppGpp synthetase in Arabidopsis (Sugliani et al , 2016).…”