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A RelA-SpoT homolog (Cr-RSH) identified in Chlamydomonas reinhardtii generates stringent factor in vivo and localizes to chloroplasts in vitro.

ABSTRACT: A gene encoding a putative guanosine 3',5'-bispyrophosphate (ppGpp) synthase-degradase, designated Cr-RSH, was identified in the unicellular photosynthetic eukaryote Chlamydomonas reinhardtii. The encoded Cr-RSH protein possesses a putative chloroplast-targeting signal at its NH2-terminus, and translocation of Cr-RSH into chloroplasts isolated from C.reinhardtii was demonstrated in vitro. The predicted mature region of Cr-RSH exhibits marked similarity to eubacterial members of the RelA-SpoT family of proteins. Expression of an NH2-terminal portion of Cr-RSH containing the putative ppGpp synthase domain in a relA, spoT double mutant of Escherichia coli complemented the growth deficits of the mutant cells. Chromatographic analysis of 32P-labeled cellular mononucleotides also revealed that expression of Cr-RSH in the mutant bacterial cells resulted in the synthesis of ppGpp. SpoT, which catalyzes (p)ppGpp degradation, is dispensable in E.coli only if cells also lack RelA, which possesses (p)ppGpp synthase activity. The complementation analysis thus indicated that Cr-RSH possesses both ppGpp synthase and degradase activities. These results represent the first demonstration of ppGpp synthase-degradase activities in a eukaryotic organism, and they suggest that eubacterial stringent control mediated by ppGpp has been conserved during evolution of the chloroplast from a photosynthetic bacterial symbiont.

SUBMITTER: Kasai K

PROVIDER: S-EPMC137175 | biostudies-literature | 2002 Nov

REPOSITORIES: biostudies-literature

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A RelA-SpoT homolog (Cr-RSH) identified in Chlamydomonas reinhardtii generates stringent factor in vivo and localizes to chloroplasts in vitro.

Kasai Koji K Usami Syoji S Yamada Takashi T Endo Yaeta Y Ochi Kozo K Tozawa Yuzuru Y

Nucleic acids research 20021101 22

A gene encoding a putative guanosine 3',5'-bispyrophosphate (ppGpp) synthase-degradase, designated Cr-RSH, was identified in the unicellular photosynthetic eukaryote Chlamydomonas reinhardtii. The encoded Cr-RSH protein possesses a putative chloroplast-targeting signal at its NH2-terminus, and translocation of Cr-RSH into chloroplasts isolated from C.reinhardtii was demonstrated in vitro. The predicted mature region of Cr-RSH exhibits marked similarity to eubacterial members of the RelA-SpoT fam ...[more]

PMID: 12434003

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Project description:RelA/SpoT Homologue (RSH) proteins, named for their sequence similarity to the RelA and SpoT enzymes of Escherichia coli, comprise a superfamily of enzymes that synthesize and/or hydrolyze the alarmone ppGpp, activator of the "stringent" response and regulator of cellular metabolism. The classical "long" RSHs Rel, RelA and SpoT with the ppGpp hydrolase, synthetase, TGS and ACT domain architecture have been found across diverse bacteria and plant chloroplasts, while dedicated single domain ppGpp-synthesizing and -hydrolyzing RSHs have also been discovered in disparate bacteria and animals respectively. However, there is considerable confusion in terms of nomenclature and no comprehensive phylogenetic and sequence analyses have previously been carried out to classify RSHs on a genomic scale. We have performed high-throughput sensitive sequence searching of over 1000 genomes from across the tree of life, in combination with phylogenetic analyses to consolidate previous ad hoc identification of diverse RSHs in different organisms and provide a much-needed unifying terminology for the field. We classify RSHs into 30 subgroups comprising three groups: long RSHs, small alarmone synthetases (SASs), and small alarmone hydrolases (SAHs). Members of nineteen previously unidentified RSH subgroups can now be studied experimentally, including previously unknown RSHs in archaea, expanding the "stringent response" to this domain of life. We have analyzed possible combinations of RSH proteins and their domains in bacterial genomes and compared RSH content with available RSH knock-out data for various organisms to determine the rules of combining RSHs. Through comparative sequence analysis of long and small RSHs, we find exposed sites limited in conservation to the long RSHs that we propose are involved in transmitting regulatory signals. Such signals may be transmitted via NTD to CTD intra-molecular interactions, or inter-molecular interactions either among individual RSH molecules or among long RSHs and other binding partners such as the ribosome.

Project description:Background:We investigated strategies to improve foreign protein accumulation in the chloroplasts of the model algae Chlamydomonas reinhardtii and tested the outcome in both standard culture conditions as well as one pertinent to algal biofuel production. The downstream box (DB) of the TetC or NPTII genes, the first 15 codons following the start codon, was N-terminally fused to the coding region of cel6A, an endoglucanase from Thermobifida fusca. We also employed a chimeric regulatory element, consisting of the 16S rRNA promoter and the atpA 5'UTR, previously reported to enhance protein expression, to regulate the expression of the TetC-cel6A gene. We further investigated the accumulation of TetC-Cel6A under N-deplete growth conditions. Results:Both of the DB fusions improved intracellular accumulation of Cel6A in transplastomic C. reinhardtii strains though the TetC DB was much more effective than the NPTII DB. Furthermore, using the chimeric regulatory element, the TetC-Cel6A protein accumulation displayed a significant increase to 0.3% total soluble protein (TSP), whereas NPTII-Cel6A remained too low to quantify. Comparable levels of TetC- and NPTII-cel6A transcripts were observed, which suggests that factors other than transcript abundance mediate the greater TetC-Cel6A accumulation. The TetC-Cel6A accumulation was stable regardless of the growth stage, and the transplastomic strain growth rate was not altered. When transplastomic cells were suspended in N-deplete medium, cellular levels of TetC-Cel6A increased over time along with TSP, and were greater than those in cells suspended in N-replete medium. Conclusions:The DB fusion holds great value as a tool to enhance foreign protein accumulation in C. reinhardtii chloroplasts and its influence is related to translation or other post-transcriptional processes. Our results also suggest that transplastomic protein production can be compatible with algal biofuel production strategies. Cells displayed a consistent accumulation of recombinant protein throughout the growth phase and nitrogen starvation, a strategy used to induce lipid production in algae, led to higher cellular heterologous protein content. The latter result is contrary to what might have been expected a priori and is an important result for the development of future algal biofuel systems, which will likely require co-products for economic sustainability.

Dataset Information

A RelA-SpoT homolog (Cr-RSH) identified in Chlamydomonas reinhardtii generates stringent factor in vivo and localizes to chloroplasts in vitro.

Publications

A RelA-SpoT homolog (Cr-RSH) identified in Chlamydomonas reinhardtii generates stringent factor in vivo and localizes to chloroplasts in vitro.

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