Biochemical properties of CumA multicopper oxidase from plant pathogen, Pseudomonas syringae

Expression of laccase and ascorbate oxidase affects lignin composition in Arabidopsis thaliana stems

Lignin is a phenolic polymer that is a major source of biomass. Oxidative enzymes, such as laccase and peroxidase, are required for lignin polymerisation. Laccase is a member of the multicopper oxidase family and has a high amino acid sequence similarity with ascorbate oxidase. However, the process of functional differentiation between the two enzymes remains poorly understood. In this study, the common ancestry sequence of laccase and ascorbate oxidase (AncMCO) was predicted via phylogenetic reconstruction, and its in vivo effect on lignin biosynthesis in Arabidopsis thaliana was assessed. The estimated AncMCO sequence conserved key residues that coordinate with copper ions, implying that the electron transfer system is likely to be conserved in AncMCO. However, multiple insertions/deletions corresponding to protein surface structures have been found between laccase, ascorbate oxidase, and AncMCO. The overexpression of canonical laccase (AtLAC4) and ascorbate oxidase (AtAAO1) in A. thaliana resulted in notable increases of syringyl/guaiacyl lignin unit ratio in stems, whereas, in contrast, the overexpression of AncMCO did not show any detectable change in lignin deposition. Transcriptomic analysis revealed that the AtAAO1-overexpressing line exhibited significant changes in the expression of a wide range of cell wall biosynthesis genes. These results highlight the importance of the molecular evolution of multicopper oxidase, which drives lignin biosynthesis during plant evolution.

Mineralogical and Genomic Constraints on the Origin of Microbial Mn Oxide Formation in Complexed Microbial Community at the Terrestrial Hot Spring

Manganese (Mn) oxides are widespread on the surface environments of the modern Earth. The role of microbial activities in the formation of Mn oxides has been discussed for several decades. However, the mechanisms of microbial Mn oxidation, and its role in complex microbial communities in natural environments, remain uncertain. Here, we report the geochemical, mineralogical, and metagenomic evidence for biogenic Mn oxides, found in Japanese hot spring sinters. The low crystallinity of Mn oxides, and their spatial associations with organic matter, support the biogenic origin of Mn oxides. Specific multicopper oxidases (MCOs), which are considered Mn-oxidizing enzymes, were identified using metagenomic analyses. Nanoscale nuggets of copper sulfides were, also, discovered in the organic matter in Mn-rich sinters. A part of these copper sulfides most likely represents traces of MCOs, and this is the first report of traces of Mn-oxidizing enzyme in geological samples. Metagenomic analyses, surprisingly, indicated a close association of Mn oxides, not only in aerobic but also in anaerobic microbial communities. These new findings offer the unique and unified positions of Mn oxides, with roles that have not been ignored, to sustain anaerobic microbial communities in hot spring environments.