Phylogenetic Analyses of Microbial Hydrolytic Dehalogenases Reveal Polyphyletic Origin

Mixotrophic cyanobacteria are critical active diazotrophs in polychlorinated biphenyl-contaminated paddy soil

Biological nitrogen fixation by diazotrophs is a crucial biogeochemical process in global terrestrial ecosystems, especially in nitrogen-limited, organic-contaminated soils. The metabolic activities of diazotrophs and their ability to supply fixed nitrogen may facilitate the transformation of organic pollutants. However, the active diazotrophic communities in organic-contaminated soils and their potential metabolic functions have received little attention. In the current study, the relationship between biological nitrogen fixation and polychlorinated biphenyl (PCB) metabolism was analyzed in situ in paddy soil contaminated with a representative tetrachlorobiphenyl (PCB52). 15N-DNA stable isotope probing was combined with high-throughput sequencing to identify active diazotrophs, which were distributed in 14 phyla, predominantly Cyanobacteria (23.40%). Subsequent metagenome binning and functional gene mining revealed that some mixotrophic cyanobacteria (e.g. FACHB-36 and Cylindrospermum) contain essential genes for nitrogen fixation, PCB metabolism, and photosynthesis. The bifunctionality of Cylindrospermum sp. in nitrogen fixation and PCB metabolism was further confirmed by metabolite analyses of Cylindrospermum sp. from a culture collection as a representative species, which showed that Cylindrospermum sp. metabolized PCB and produced 2-chlorobiphenyl and 2,5-dihydroxybenzonic acid. Collectively, these findings indicate that active diazotrophs, particularly mixotrophic cyanobacteria, have important ecological remediation functions and are a promising nature-based in situ remediation solution for organic-contaminated environments.

Phylogenetic analyses reveal insights into interdomain horizontal gene transfer of microbial lipases

Microbial lipases play a pivotal role in a wide range of biotechnological processes and in the human skin microbiome. However, their evolution remains poorly understood. Accessing the evolutionary process of lipases could contribute to future applications in health and biotechnology. We investigated genetic events associated with the evolutionary trajectory of the microbial family LIP lipases. Using phylogenetic analysis, we identified two distinct horizontal gene transfer (HGT) events from Bacteria to Fungi. Further analysis of human cutaneous mycobiome members such as the lipophilic Malassezia yeasts and CUG-Ser-1 clade (including Candida sp. and other microorganisms associated with cutaneous mycobiota) revealed recent evolutionary processes, with multiple gene duplication events. The Lid region of fungal lipases, crucial for substrate interaction, exhibits varying degrees of conservation among different groups. Our findings suggest the adaptability of the fungal LIP family in various genetic and metabolic contexts and its potential role in niche exploration.

Catalytic role of histidine-114 in the hydrolytic dehalogenation of chlorothalonil by Pseudomonas sp. CTN-3

Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile; TPN) is an environmentally persistent fungicide that sees heavy use in the USA and is highly toxic to aquatic species and birds, as well as a probable human carcinogen. The chlorothalonil dehalogenase from Pseudomonas sp. CTN-3 (Chd, UniProtKB C9EBR5) degrades TPN to its less toxic 4-OH-TPN analog making it an exciting candidate for the development of a bioremediation process for TPN; however, little is currently known about its catalytic mechanism. Therefore, an active site residue histidine-114 (His114) which forms a hydrogen bond with the Zn(II)-bound water/hydroxide and has been suggested to be the active site acid/base, was substituted by an Ala residue. Surprisingly, ChdH114A exhibited catalytic activity with a kcat value of 1.07 s-1, ~ 5% of wild-type (WT) Chd, and a KM of 32 µM. Thus, His114 is catalytically important but not essential. The electronic and structural aspects of the WT Chd and ChdH114A active sites were examined using UV-Vis and EPR spectroscopy on the catalytically competent Co(II)-substituted enzyme as well as all-atomistic molecular dynamics (MD) simulations. Combination of these data suggest His114 can quickly and reversibly move nearly 2 Å between one conformation that facilitates catalysis and another that enables product egress and active site recharge. In light of experimental and computational data on ChdH114A, Asn216 appears to play a role in substrate binding and preorganization of the transition-state while Asp116 likely facilitates the deprotonation of the Zn(II)-bound water in the absence of His114. Based on these data, an updated proposed catalytic mechanism for Chd is presented.