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Harmful cyanobacterial blooms are a growing environmental problem worldwide in natural waters,the biodegradation is found to be the most efficient method for removing microcystins (MCs) produced by harmful cyanobacteria.Based on the isolation of a promising bacterial strain of Sphingopyxis sp.USTB-05 for biodegrading MCs,we for the first time cloned and expressed a gene USTB-05-A (HM245411) that is responsible for the first step in the biodegradation of microcystin LR (MC-LR) in E.coli DH5α,with a cloning vector of pGEM-T easy and an expression vector of pGEX-4T-1,respectively.The cell-free extracts (CE) of recombinant E.coli DH5α containing USTB-05-A had high activity for biodegrading MC-LR.The initial MC-LR concentration of 40 mg/L was completely biodegraded within 1 hr in the presence of CE with a protein concentration of 0.35 mg/mL.Based on an analysis of the liquid chromatogram-mass spectrum (LC-MS),the enzyme encoded by gene USTB-05-A was found to be active in cleaving the target peptide bond between 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-deca-4,6-dienoic acid (Adda) and arginine of MC-LR,and converting cyclic MC-LR to linear MC-LR as a first product that is much less toxic than parent MC-LR,which offered direct evidence for the first step on the pathway of MC-LR biodegradation by Sphingopyxis sp.USTB-05.