Degradation Activity Of A MCs Degrading Strain And The Evolutionary Origin Of The Mlr Gene Cluster

Microcystins(MCs)are hepatotoxic cyclic heptapeptides produced by cyanobacteria.The mlr-dependent pathway plays a crucial role in the biodegradation of MCs.However,due to the lack of complete sequence of the mlr gene cluster,the evolutionary origin of the mlr-dependent pathway is still unclear.In this study,a strain of MCs-degrading bacteria was isolated and identified from the sediment of Chaohu Lake.Then the degradation characteristics of MCLR and the influence of environmental factors on the degradation ability of MCLR were studied.The partial sequence of the mlr gene cluster of the degrading bacteria was amplified and sequenced by PCR,then the evolutionary origin of the mlr gene cluster was analyzed by combining the reported mlr gene sequence.Specific research contents and results are as follows:(1)A MCs-degrading strain,CHF4,was isolated from the sediments of Chaohu Lake by dilution coating.16 S rDNA identification showed that the strain was Sphingosinicella sp.The results of PCR showed that the strain contained all four genes of the mlr gene cluster,which proved that it adopted the mlr pathway.(2)Strain CHF4 have high degradation ability at 25-35 ? and pH 7-9.The maximum degradation rate was 0.54 mg·L-1h-1,higher than most isolated MCsdegrading bacteria.(3)Part of the mlr gene cluster with length of 5418 bp was amplified by PCR,which contained complete sequences of mlrA,mlrB and mlrD genes.The similarity of mlr gene cluster between strain CHF4 and other MCs-degrading bacteria was more than 86%,suggesting that the mlr gene cluster may have the same evolutionary origin.(4)Phylogenetic analysis of the mlr gene cluster showed that the four genes of the mlr gene cluster had the same origin and evolution history.Genomic island analysis showed that the mlr gene cluster of MCs-degrading bacteria in sphingomonads was located on the same genomic island.Combined with the distribution of mlr gene cluster in MCS-degrading bacteria,mlr gene cluster may initially be transferred into sphingomonads gene pool through horizontal gene transfer,and then into its subgenus through vertical inheritance.(5)The comparative results of mlr gene clusters and the distribution of mlr gene clusters in non-sphingomonads bacteria indicate that strains other than sphingomonads may acquire mlr genes from sphingomonads gene pool through recent horizontal gene transfer.The results of this study deepen the understanding of the evolutionary origin of mlr gene cluster,not only reveal the distribution of mlr gene cluster in bacteria,but also help to understand the whole mlr pathway,which is of great significance for environmental risk assessment of MCs pollution.