No abstract

The Arabidopsis endonuclease RNase E (RNE) is localized in the chloroplast and is involved in processing of plastid ribonucleic acids (RNAs). By expression of a tandem affinity purification-tagged version of the plastid RNE in the Arabidopsis rne mutant background in combination with mass spectrometry, we identified the novel vascular plant-specific and co-regulated interaction partner of RNE, designated RHON1. RHON1 is essential for photoautotrophic growth and together with RNE forms a distinct ∼800 kDa complex. Additionally, RHON1 is part of various smaller RNA-containing complexes. RIP-chip and other association studies revealed that a helix-extended-helix-structured Rho-N motif at the C-terminus of RHON1 binds to and supports processing of specific plastid RNAs. In all respects, such as plastid RNA precursor accumulation, protein pattern, increased number and decreased size of chloroplasts and defective chloroplast development, the phenotype of rhon1 knockout mutants resembles that of rne lines. This strongly suggests that RHON1 supports RNE functions presumably by conferring sequence specificity to the endonuclease.

Synthesis of the D1 reaction center protein of Photosystem II is dynamically regulated in response to environmental and developmental cues. In chloroplasts, much of this regulation occurs at the post-transcriptional level, but the proteins responsible are largely unknown. To discover proteins that impact psbA expression, we identified proteins that associate with maize psbA mRNA by: (i) formaldehyde cross-linking of leaf tissue followed by antisense oligonucleotide affinity capture of psbA mRNA; and (ii) co-immunoprecipitation with HCF173, a psbA translational activator that is known to bind psbA mRNA. The S1 domain protein SRRP1 and two RNA Recognition Motif (RRM) domain proteins, CP33C and CP33B, were enriched with both approaches. Orthologous proteins were also among the enriched protein set in a previous study in Arabidopsis that employed a designer RNA-binding protein as a psbA RNA affinity tag. We show here that CP33B is bound to psbA mRNA in vivo, as was shown previously for CP33C and SRRP1. Immunoblot, pulse labeling, and ribosome profiling analyses of mutants lacking CP33B and/or CP33C detected some decreases in D1 protein levels under some conditions, but no change in psbA RNA abundance or translation. However, analogous experiments showed that SRRP1 represses psbA ribosome association in the dark, represses ycf1 ribosome association, and promotes accumulation of ndhC mRNA. As SRRP1 is known to harbor RNA chaperone activity, we postulate that SRRP1 mediates these effects by modulating RNA structures. The uncharacterized proteins that emerged from our analyses provide a resource for the discovery of proteins that impact the expression of psbA and other chloroplast genes.psbA mRNA-associated proteome 371 d Cutoff: >3-fold enrichment in the designer SCD11 pull-down data (McDermott et al., 2019). Proteins not found in the chloroplast were excluded. *Not detected in control (pre-immune serum for HCF173 co-immunoprecipitates, leaf proteome for PIRP, anti-FLAG pull-down of Col-0 for dPPR).

SUMMARYChloroplast RNA metabolism depends on a multitude of nuclear-encoded RNA-binding proteins (RBPs). Most known chloroplast RBPs address specific RNA targets and RNA-processing functions. However, members of the small chloroplast ribonucleoprotein family (cpRNPs) play a global role in processing and stabilizing chloroplast RNAs. Here, we show that the cpRNP CP33A localizes to a distinct sub-chloroplastic domain and is essential for chloroplast development. The loss of CP33A yields albino seedlings that exhibit aberrant leaf development and can only survive in the presence of an external carbon source. Genome-wide RNA association studies demonstrate that CP33A associates with all chloroplast mRNAs. For a given transcript, quantification of CP33A-bound versus free RNAs demonstrates that CP33A associates with the majority of most mRNAs analyzed. Our results further show that CP33A is required for the accumulation of a number of tested mRNAs, and is particularly relevant for unspliced and unprocessed precursor mRNAs. Finally, CP33A fails to associate with polysomes or to strongly co-precipitate with ribosomal RNA, suggesting that it defines a ribodomain that is separate from the chloroplast translation machinery. Collectively, these findings suggest that CP33A contributes to globally essential RNA processes in the chloroplasts of higher plants.

Chloroplast RNAs are stabilized and processed by a multitude of nuclear-encoded RNA-binding proteins, often in response to external stimuli like light and temperature. A particularly interesting RNA-based regulation occurs with the psbA mRNA, which shows light-dependent translation. Recently, the chloroplast ribonucleoprotein CP33B was identified as a ligand of the psbA mRNA. We here characterized the interaction of CP33B with chloroplast RNAs in greater detail using a combination of RIP-chip, quantitative dot-blot, and RNA-Bind-n-Seq experiments. We demonstrate that CP33B prefers psbA over all other chloroplast RNAs and associates with the vast majority of the psbA transcript pool. The RNA sequence target motif, determined in vitro, does not fully explain CP33B's preference for psbA, suggesting that there are other determinants of specificity in vivo.Plants 2020, 9, 367 2 of 15 of target mRNAs for various cpRNPs [7][8][9]. rRNAs and intron-less tRNAs are not or only weakly bound [6,7,10]. Since the cpRNPs do not co-fractionate with polysomal RNAs [6,8,11], they are mainly attributed a function prior to translation within posttranscriptional processes.Prediction algorithms for subcellular localization and shotgun proteome analysis identified all ten cpRNPs of Arabidopsis in the chloroplast [summarized in 2]. Fluorescence microscopy of GFP fusion proteins confirmed the chloroplast localization [8,[12][13][14]. Within the chloroplasts, the stroma is the main destination of cpRNPs, with small amounts also being associated with thylakoids. This was proven by immunological analyses for the five cpRNPs from tobacco [6].The expression of cpRNPs is regulated by various external and internal signals. Light especially leads to an accumulation of cpRNPs [summarized in 2]. In general, cpRNPs are involved in a variety of posttranscriptional processes, including 3'-end processing of RNAs [15], RNA editing [16,17], RNA splicing [7], and RNA stabilization [7,8,10]. Some of these processes are modulated by cpRNPs in response to environmental cues and several cpRNPs have been implicated in different acclimation and stress responses [2,7,13,18]. Such a multi-level and far-reaching regulation by multiple external and internal stimuli is unknown for most other chloroplast RBPs, including PPR proteins. cpRNPs are thus considered as prime candidates for post-transcriptional regulators of plastid gene expression [19].A particularly interesting case of chloroplast gene regulation is the light-induced translation of psbA, which codes for the D1 protein, the core subunit of photosytem II [20][21][22][23]. D1 is constantly damaged, most pronouncedly by excess light and other unfavourable conditions, i.e., cold. As a consequence, D1 is constantly synthesized for the repair of PSII [24][25][26]. Moreover, regulated D1 synthesis for de novo biogenesis of PSII during cell growth requires additional regulatory levels of psbA mRNA translation. Consistently, a number of proteins have been co-purified with the psbA mRNA in Chlamydomonas, spinach, A...