Elucidating the Regulatory Functions of MlrA Originated from Novosphingobium sp. THN1 in Microcystin-LR Degradation

Abstract

Microcystin-LR (MC-LR), produced by harmful cyanobacteria, seriously endangers animals and humans. Biodegradation appears as the major pathway for natural MC-LR attenuation. To elucidate the regulatory function of mlrA gene of Novosphingobium sp. THN1 (i.e., THN1-mlrA gene) in MC-LR biodegradation, this study constructed a recombinant bacterium and succeeded in heterlogously expressing the mlrA of THN1 strain (i.e., THN1-MlrA enzyme). The recombinant mlrA exhibited the activity for smoothly degrading 20 μg mL-1 of MC-LR at an average rate of 0.16 μg mL-1 h-1 within 80 h. Mass spectrum analysis confirmed that recombinant mlrA hydrolyzed cyclic MC-LR by cleaving the peptide bond between Adda and arginine residue and generated linearized MC-LR as primary intermediate. Such linearization for MC-LR catalyzed by THN1-MlrA enzyme was particularly important during MC-LR biodegradation process, because it opened the highly-stable cyclic structure of MC-LR and caused substantial detoxification. These findings for the first time manifested that mlrA gene homolog of Novosphingobium genus conserved its original catalytic function as described elsewhere. This study expanded the knowledge on the function of mlrA homologs from various natural habitats, and facilitated the understanding on the fate and biological attenuation mechanisms of MC-LR in Lake Taihu, China, where THN1 strain is indigenous.