Abstract
Bacterial ribosome hibernation factors sequester ribosomes in an inactive state during the stationary phase and in response to stress. The cyanobacterial ribosome hibernation factor LrtA has been suggested to inactivate ribosomes in the dark and to be important for post-stress survival. In this study, we addressed the hypothesis that Plastid Specific Ribosomal Protein 1 (PSRP1), the chloroplast-localized LrtA homolog in plants, contributes to the global repression of chloroplast translation that occurs when plants are shifted from light to dark. We found that the abundance of PSRP1 and its association with ribosomes were similar in the light and the dark. Maize mutants lacking PSRP1 were phenotypically normal under standard laboratory growth conditions. Furthermore, the absence of PSRP1 did not alter the distribution of chloroplast ribosomes among monosomes and polysomes in the light or in the dark, and did not affect the light-regulated synthesis of the chloroplast psbA gene product. These results suggest that PSRP1 does not play a significant role in the regulation of chloroplast translation by light. As such, the physiological driving force for the retention of PSRP1 during chloroplast evolution remains unclear.
Highlights
In photosynthetic eukaryotes, photosynthesis takes place inside chloroplasts, specialized organelles descended from cyanobacteria
We address the hypothesis that Plastid Specific Ribosomal Protein 1 (PSRP1) globally represses chloroplast protein synthesis in the dark
PSRP1 emerged as a candidate factor that contributes to this global shut down of translation because it binds and inactivates 70S ribosomes, and its cyanobacterial homologs are induced in the dark because it binds and inactivates 70S ribosomes, and its cyanobacterial homologs are induced in the and decrease the fraction of ribosomes found in polysomes [6,8,9,14]
Summary
Photosynthesis takes place inside chloroplasts, specialized organelles descended from cyanobacteria. Many of the multimeric complexes involved in photosynthesis and chloroplast gene expression are of dual genetic origin, in that some subunits are encoded in the chloroplast genome and others in the nuclear genome. The average size of cytosolic polysomes decreases shortly after shifting plants from light to dark [3,4], and translation in chloroplasts is globally repressed in the dark at the elongation and initiation steps [4]. The mechanisms underlying this global regulation of chloroplast translation in response to light are not known
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