Chloroplast and mitochondrial DNA editing in plants

Plant organelles including mitochondria and chloroplasts contain their own genomes, which encode many genes essential for respiration and photosynthesis, respectively. Gene editing in plant organelles, an unmet need for plant genetics and biotechnology, has been hampered by the lack of appropriate tools for targeting DNA in these organelles. In this study, we developed a Golden Gate cloning system1, composed of 16 expression plasmids (8 for the delivery of the resulting protein to mitochondria and the other 8 for delivery to chloroplasts) and 424 transcription activator-like effector subarray plasmids, to assemble DddA-derived cytosine base editor (DdCBE)2 plasmids and used the resulting DdCBEs to efficiently promote point mutagenesis in mitochondria and chloroplasts. Our DdCBEs induced base editing in lettuce or rapeseed calli at frequencies of up to 25% (mitochondria) and 38% (chloroplasts). We also showed DNA-free base editing in chloroplasts by delivering DdCBE mRNA to lettuce protoplasts to avoid off-target mutations caused by DdCBE-encoding plasmids. Furthermore, we generated lettuce calli and plantlets with edit frequencies of up to 99%, which were resistant to streptomycin or spectinomycin, by introducing a point mutation in the chloroplast 16S rRNA gene.

A conserved BAH module within mammalian BAHD1 connects H3K27me3 to Polycomb gene silencing

ABSTRACTTrimethylation of histone H3 lysine 27 (H3K27me3) is important for gene silencing and imprinting, (epi)genome organization and organismal development. In a prevalent model, the functional readout of H3K27me3 in mammalian cells is achieved through the H3K27me3-recognizing chromodomain harbored within the chromobox (CBX) component of canonical Polycomb repressive complex 1 (cPRC1), which induces chromatin compaction and gene repression. Here, we report that binding of H3K27me3 by a Bromo Adjacent Homology (BAH) domain harbored within BAH domain-containing protein 1 (BAHD1) is required for overall BAHD1 targeting to chromatin and for optimal repression of the H3K27me3-demarcated genes in mammalian cells. Disruption of direct interaction between BAHD1BAH and H3K27me3 by point mutagenesis leads to chromatin remodeling, notably, increased histone acetylation, at its Polycomb gene targets. Mice carrying an H3K27me3-interaction-defective mutation of Bahd1BAH causes marked embryonic lethality, showing a requirement of this pathway for normal development. Altogether, this work demonstrates an H3K27me3-initiated signaling cascade that operates through a conserved BAH “reader” module within BAHD1 in mammals.Key PointsBAHD1BAH is a functionally validated mammalian “reader” of H3K27me3, mediating BAHD1 targeting for gene silencing.BAHD1BAH connects H3K27me3 together with histone deacetylation, an integral step of gene silencing.BAHD1BAH-mediated functional readout of H3K27me3 is essential for organismal development.Graphic abstractA mammalian H3K27me3-transduction pathway operates through an H3K27me3-specific ‘reader’ module (BAH) of BAHD1, which assembles a complex with corepressors (HDACs and others) for suppressing histone acetylation and repressing expression at Polycomb target genes.

Chloroplast and mitochondrial DNA editing in plants

Plant organelles, including mitochondria and chloroplasts, contain their own genomes, which encode hundreds of genes essential for respiration and photosynthesis, respectively. Gene editing in plant organelles, an unmet need for plant genetics and biotechnology, has been hampered by the lack of appropriate tools for targeting DNA in these organelles. In this study, we developed a Golden Gate cloning system, composed of 16 expression plasmids (8 for delivery of the resulting protein to mitochondria and the other 8 for delivery to chloroplasts) and 424 TALE sub-array plasmids, to assemble DddA-derived cytosine base editor (DdCBE) plasmids and used the resulting DdCBEs to promote point mutagenesis in mitochondria and chloroplasts efficiently. Our DdCBEs induced base editing in lettuce or rapeseed calli at frequencies of up to 25% (mitochondria) and 38% (chloroplasts). We also showed DNA-free base editing in chloroplasts by delivering DdCBE mRNA to lettuce protoplasts. Furthermore, we generated lettuce calli resistant to streptomycin, an antibiotic that binds to 16S ribosomal RNA (rRNA) irreversibly, leading to inhibition of protein synthesis, by introducing a point mutation in the chloroplast 16S rRNA gene.

Plasmodium falciparum guanylyl cyclase-alpha and the activity of its appended P4-ATPase domain are essential for cGMP synthesis and blood stage egress

In malaria parasites, guanylyl cyclases (GCs), which synthesise cyclic GMP (cGMP), are associated with a P4-ATPase-like domain in a unique bifunctional configuration. P4-ATPases generate membrane bilayer lipid asymmetry by translocating phospholipids from the outer to the inner leaflet. Here we investigate the role of Plasmodium falciparum guanylyl cyclase alpha (GCα) and its associated P4-ATPase module, showing that asexual blood stage parasites lacking both the cyclase and P4-ATPase domains are unable to egress from host erythrocytes. GCα-null parasites cannot synthesise cGMP, or mobilise calcium, a cGMP-dependent protein kinase (PKG)-driven requirement for egress. Using chemical complementation with a cGMP analogue and point mutagenesis of a crucial conserved residue within the P4-ATPase domain, we show that ATPase activity is up stream of and linked to cGMP synthesis. Collectively, our results demonstrate that GCα is a critical regulator of PKG and that its associated P4-ATPase domain plays a primary role in generating cGMP for merozoite egress.

In vitro Evolution of Antibody Affinity via Insertional Mutagenesis Scanning of an Entire Antibody Variable Region

We report the first systematic combinatorial exploration of affinity enhancement of antibodies by insertions and deletions (InDels). Transposon-based introduction of InDels via the method TRIAD was used to generate large libraries with random in-frame InDels across the entire scFv gene that were further recombined and screened by ribosome display. Knowledge of potential insertion points from TRIAD libraries formed the basis of exploration of length and sequence diversity of novel insertions by insertional-scanning mutagenesis (ISM). An overall 256-fold affinity improvement of an anti-IL-13 antibody BAK1 as a result of InDel mutagenesis and combination with known point mutations validates this approach and suggests that the results of this InDel approach and conventional exploration of point mutations can synergize to generate antibodies with higher affinity.SignificanceInsertion/deletion (InDel) mutations play key roles in genome and protein evolution. Despite their prominence in evolutionary history, the potential of InDels for changing function in protein engineering by directed evolution remains unexplored. Instead point mutagenesis is widely used. Here we create antibody libraries containing InDels and demonstrate that affinity maturation can be achieved in this way, establishing an alternative to the point mutation strategies employed in all previous in vitro selections. These InDels mirror the observation of considerable length variation in loops of natural antibodies originating from the same germline genes and be combined with point mutations, making both natural sources of functional innovation available for artificial evolution in the test tube.

Access to unexplored regions of sequence space in directed enzyme evolution via insertion/deletion mutagenesis

ABSTRACTInsertions and deletions (InDels) are frequently observed in natural protein evolution, yet their potential remains untapped in laboratory evolution. Here we introduce a transposon mutagenesis approach (TRIAD) to generate libraries of random variants with short in-frame InDels, and screen TRIAD libraries to evolve a promiscuous arylesterase activity in a phosphotriesterase. The evolution exhibits features that are distinct from previous point mutagenesis campaigns: while the average activity of TRIAD variants is more deleterious, a larger proportion has successfully adapted for the new activity, exhibiting different functional profiles: (i) both strong and weak trade-off in original vs promiscuous activity are observed; (ii) trade-off is more severe (10- to 20-fold increased kcat/KM in arylesterase with ∼100-fold decreases in the original phosphotriesterase activity) and (iii) improvements show up in kcat rather than KM, suggesting novel adaptive solution. These distinct features make TRIAD an alternative to widely used point mutagenesis, providing access to functional innovations and traversing unexplored fitness landscape regions.

Site‐directed mutagenesis, in vivo electroporation and mass spectrometry in search for determinants of the subcellular targeting of Rab7b paralogue in the model eukaryote

<p>Protein products of the paralogous genes resulting from the whole genome duplication may acquire new function. The role of post‐translational modifications (PTM) in proper targeting of <em>Paramecium</em> Rab7b paralogue – distinct from that of Rab7a directly involved in phagocytosis ‐ was studied using point mutagenesis, proteomic analysis and double immunofluorescence after <em>in vivo</em> electroporation of the mutagenized protein. Here we show that substitution of Thr200 by Ala200 resulted in diminished incorporation of [P³²] by 37.4% and of 32 [^C14^–]UDP‐glucose by 24%, respectively, into recombinant Rab7b_200 in comparison to the non‐mutagenized control. Double confocal imaging revealed that Rab7b_200 was mistargeted upon electroporation into living cells contrary to non‐ mutagenized recombinant Rab7b correctly incorporated in the cytostome area. We identified the peptide ion at m/z=677.6³⁺ characteristic for the glycan group attached to Thr200 in Rab7b using nano LC‐MS/MS and comparing the peptide map of this protein with that after deglycosylation with the mixture of five enzymes of different specificity. Based on the mass of this peptide ion and quantitative radioactive assays with [P³²]and [C¹⁴^‐]UDP‐ glucose, the suggested composition of the adduct attached to Thr200 might be (Hex)1(HexNAc)1(Phos)3 or (HexNAc)1 (Deoxyhexose)1 (Phos)1 (HexA)1. These data indicate that PTM of Thr200 located in the hypervariable C‐region of Rab7b in <em>Paramecium</em> is crucial for the proper localization/function of this protein. Moreover, these proteins differ also in other PTM: the number of phosphorylated amino acids in Rab7b is much higher than in Rab7a. </p>