Comprehensive Insight into Microcystin-Degrading Mechanism of Sphingopyxis sp. m6 Based on Mlr Enzymes

Bacterial degradation is one important Microcystin (MC) removal method in the natural environment. The traditional MC-degrading pathway was proposed based on the functions of individual recombinant Mlr enzymes and the structures of the main MC-degrading products. However, the actual MC-degrading mechanism by Mlr enzymes in wild-type bacteria remains unclear. In this study, bioinformatic analysis, heterologous expression, and knockout mutation were performed to elaborate the MC-degrading mechanism by Mlr enzymes in Sphingopyxis sp. m6. The results showed that mlr gene cluster was initially acquired by horizontal gene transfer, followed by vertical inheritance within Alphaproteobacteria. Mlr enzymes exhibit distinct subcellular localizations and possess diverse conserved catalytic domains. The enzymatic cascade MlrA/MlrB/MlrC sequentially cleaves Microcystin-LR (MC-LR) via Adda-Arg, Ala-Leu, and Adda-Glu bonds, generating characteristic intermediates (linearized MC-LR, tetrapeptide, and Adda). Notably, recombinant MlrC demonstrated dual-targeting degrading capability (linearized MC-LR and tetrapeptide), while tetrapeptide specificity in endogenous processing of Sphingopyxis sp. m6. Marker-free knockout mutants of mlr genes were first constructed in MC-degrading bacteria, unveiling that mlrA was indispensable in initial MC cleavage, whereas mlrB/mlrC/mlrD displayed functional compensation through other enzymes with similar functions. This study promotes the mechanistic understanding of MC bacterial degradation and offers a theoretical basis for a bioremediation strategy targeting cyanotoxin pollution.