Search for: All records

SUMMARY Translation of the chloroplastpsbAmRNA in angiosperms is activated by photodamage of its gene product, the D1 subunit of photosystem II (PSII), providing nascent D1 for PSII repair. The involvement of chlorophyll in the regulatory mechanism has been suggested due to the regulatory roles of proteins proposed to mediate chlorophyll/D1 transactions and the fact that chlorophyll is synthesized only in the light in angiosperms. We used ribosome profiling and RNA‐seq to address whether the effects of light on chloroplast translation are conserved in the liverwort Marchantia (Marchantia polymorpha), which synthesizes chlorophyll in both the dark and the light. As in angiosperms, ribosome occupancy onpsbAmRNA decreased rapidly upon shifting plants to the dark and was rapidly restored upon a transfer back to the light, whereas ribosome occupancy on other chloroplast mRNAs changed very little. The results were similar in aMarchantiamutant unable to synthesize chlorophyll in the dark. Those results, in conjunction with pulse‐labeling data, suggest that light elicits a plastome‐wide activation of translation elongation and a specific increase inpsbAtranslation initiation inMarchantia, as in angiosperms. These findings show that light regulates chloroplast translation similarly in vascular and non‐vascular plants, and that constitutive chlorophyll synthesis does not affect light‐regulatedpsbAtranslation initiation. Additionally, the translational outputs of chloroplast genes are similar inMarchantiaand angiosperms but result from differing contributions of mRNA abundance and translational efficiencies. This adds to the evidence that chloroplast mRNA abundance and translational efficiencies co‐evolve under selection to maintain protein outputs. 

Abstract Translation initiation on chloroplast psbA mRNA in plants scales with light intensity, providing its gene product, D1, as needed to replace photodamaged D1 in Photosystem II. The psbA translational activator HIGH CHLOROPHYLL FLUORESCENCE 173 (HCF173) has been hypothesized to mediate this regulation. HCF173 belongs to the short-chain dehydrogenase/reductase superfamily, associates with the psbA 5′-untranslated region (5′-UTR), and has been hypothesized to enhance translation by binding an RNA segment that would otherwise pair with and mask the ribosome binding region. To test these hypotheses, we examined whether a synthetic pentatricopeptide repeat (sPPR) protein can substitute for HCF173 when bound to the HCF173 binding site. We show that an sPPR designed to bind HCF173's footprint in the psbA 5′-UTR bound the intended site in vivo and partially substituted for HCF173 to activate psbA translation. However, sPPR-activated translation did not respond to light. These results imply that HCF173 activates translation, at least in part, by sequestering the RNA it binds to maintain an accessible ribosome binding region, and that HCF173 is also required to regulate psbA translation in response to light. Translational activation can be added to the functions that can be programmed with sPPR proteins for synthetic biology applications in chloroplasts.