Biochemical Characterization and Structural Basis of Reactivity and Regioselectivity Differences between Burkholderia thailandensis and Burkholderia glumae 1,6-Didesmethyltoxoflavin N-Methyltransferase

Crystal structure of human METTL6, the m3C methyltransferase

In tRNA, the epigenetic m³C modification at position 32 in the anticodon loop is highly conserved in eukaryotes, which maintains the folding and basepairing functions of the anticodon. However, the responsible enzymes METTL2 and METTL6 were identified only in recent years. The loss of human METTL6 (hMETTL6) affects the translational process and proteostasis in cells, while in mESCs cells, it leads to defective pluripotency potential. Despite its important functions, the catalytic mechanism of the C32 methylation by this enzyme is poorly understood. Here we present the 1.9 Å high-resolution crystal structure of hMETTL6 bound by SAH. The key residues interacting with the ligand were identified and their roles were confirmed by ITC. We generated a docking model for the hMETTL6-SAH-CMP ternary complex. Interestingly, the CMP molecule binds into a cavity in a positive patch with the base ring pointing to the inside, suggesting a flipped-base mechanism for methylation. We further generated a model for the quaternary complex with tRNA^Ser as a component, which reasonably explained the biochemical behaviors of hMETTL6. Taken together, our crystallographic and biochemical studies provide important insight into the molecular recognition mechanism by METTL6 and may aid in the METTL-based rational drug design in the future.

Characterization of the N-methyltransferases involved in the biosynthesis of toxoflavin, fervenulin and reumycin from Streptomyces hiroshimensis ATCC53615

Toxoflavin (1), fervenulin (2), and reumycin (3), known to be produced by plant pathogen Burkholderia glumae BGR1, are structurally related 7-azapteridine antibiotics. Previous biosynthetic studies revealed that N-methyltransferase ToxA from B. glumae BGR1 catalyzed the sequential methylation at N6 and N1 in pyrimido[5,4-e]-as-triazine-5,7(6H,8H)-dione (4) to generate 1. However, the N8 methylation of 4 in the biosynthesis of fervenulin remains unclear. To explore the N-methyltransferases required for the biosynthesis of 1 and 2, we identified and characterized the fervenulin and toxoflavin biosynthetic gene clusters in S. hiroshimensis ATCC53615. On the basis of the structures of intermediates accumulated from the four N-methyltransferase gene inactivation mutants and systematic enzymatic methylation reactions, the tailoring steps for the methylation order in the biosynthesis of 1 and 2 were proposed. The N-methylation order and routes for the biosynthesis of fervenulin and toxoflavin in S. hiroshimensis are more complex and represent an obvious departure from those in B. glumae BGR1.

A genomics perspective on natural product biosynthesis in plant pathogenic bacteria

This review summarizes findings from genomics-inspired natural product research in plant pathogenic bacteria and discusses emerging trends in this field.