Recurrent requirement for the m6A-ECT2/ECT3/ECT4 axis in the control of cell proliferation during plant organogenesis.

mRNA methylation at the N6-position of adenosine (m6A) enables multiple layers of post-transcriptional gene control, often via RNA-binding proteins that use a YT521-B homology (YTH) domain for specific m6A recognition. In Arabidopsis, normal leaf morphogenesis and rate of leaf formation require m6A and the YTH-domain proteins ECT2, ECT3 and ECT4. In this study, we show that ect2/ect3 and ect2/ect3/ect4 mutants also exhibit slow root and stem growth, slow flower formation, defective directionality of root growth, and aberrant flower and fruit morphology. In all cases, the m6A-binding site of ECT proteins is required for in vivo function. We also demonstrate that both m6A methyltransferase mutants and ect2/ect3/ect4 exhibit aberrant floral phyllotaxis. Consistent with the delayed organogenesis phenotypes, we observe particularly high expression of ECT2, ECT3 and ECT4 in rapidly dividing cells of organ primordia. Accordingly, ect2/ect3/ect4 mutants exhibit decreased rates of cell division in leaf and vascular primordia. Thus, the m6A-ECT2/ECT3/ECT4 axis is employed as a recurrent module to stimulate plant organogenesis, at least in part by enabling rapid cellular proliferation.

Drosophila melanogaster nonribosomal peptide synthetase Ebony encodes an atypical condensation domain.

The protein Ebony from Drosophila melanogaster plays a central role in the regulation of histamine and dopamine in various tissues through condensation of these amines with ß-alanine. Ebony is a rare example of a nonribosomal peptide synthetase (NRPS) from a higher eukaryote and contains a C-terminal sequence that does not correspond to any previously characterized NRPS domain. We have structurally characterized this C-terminal domain and have discovered that it adopts the aryl-alkylamine-N-acetyl transferase (AANAT) fold, which is unprecedented in NRPS biology. Through analysis of ligand-bound structures, activity assays, and binding measurements, we have determined how this atypical condensation domain is able to provide selectivity for both the carrier protein-bound amino acid and the amine substrates, a situation that remains unclear for standard condensation domains identified to date from NRPS assembly lines. These results demonstrate that the C terminus of Ebony encodes a eukaryotic example of an alternative type of NRPS condensation domain; they also illustrate how the catalytic components of such assembly lines are significantly more diverse than a minimal set of conserved functional domains.

An m6A-YTH Module Controls Developmental Timing and Morphogenesis in Arabidopsis.

Methylation of N6-adenosine (m6A) in mRNA is an important posttranscriptional gene regulatory mechanism in eukaryotes. m6A provides a binding site for effector proteins ("readers") that influence pre-mRNA splicing, mRNA degradation, or translational efficiency. YT521-B homology (YTH) domain proteins are important m6A readers with established functions in animals. Plants contain more YTH domain proteins than other eukaryotes, but their biological importance remains unknown. Here, we show that the cytoplasmic Arabidopsis thaliana YTH domain proteins EVOLUTIONARILY CONSERVED C-TERMINAL REGION2/3 (ECT2/3) are required for the correct timing of leaf formation and for normal leaf morphology. These functions depend fully on intact m6A binding sites of ECT2 and ECT3, indicating that they function as m6A readers. Mutation of the close ECT2 homolog, ECT4, enhances the delayed leaf emergence and leaf morphology defects of ect2/ect3 mutants, and all three ECT proteins are expressed at leaf formation sites in the shoot apex of young seedlings and in the division zone of developing leaves. ECT2 and ECT3 are also highly expressed at early stages of trichome development and are required for trichome morphology, as previously reported for m6A itself. Overall, our study establishes the relevance of a cytoplasmic m6A-YTH regulatory module in the timing and execution of plant organogenesis.

Diversity of nature's assembly lines - recent discoveries in non-ribosomal peptide synthesis.

The biosynthesis of complex natural products by non-ribosomal peptide synthetases (NRPSs) and the related polyketide synthases (PKSs) represents a major source of important bioactive compounds. These large, multi-domain machineries are able to produce a fascinating range of molecules due to the nature of their modular architectures, which allows natural products to be assembled and tailored in a modular, step-wise fashion. In recent years there has been significant progress in characterising the important domains and underlying mechanisms of non-ribosomal peptide synthesis. More significantly, several studies have uncovered important examples of novel activity in many NRPS domains. These discoveries not only greatly increase the structural diversity of the possible products of NRPS machineries but - possibly more importantly - they improve our understanding of what is a highly important, yet complex, biosynthetic apparatus. In this review, several recent examples of novel NRPS function will be introduced, which highlight the range of previously uncharacterised activities that have now been detected in the biosynthesis of important natural products by these mega-enzyme synthetases.