Isolation of a Novel Microcystin-Degrading Bacterium and the Evolutionary Origin of Gene Cluster.

The -dependent biodegradation plays an essential role in the natural attenuation of microcystins (MCs) in eutrophic freshwater ecosystems. However, their evolutionary origin is still unclear due to the lack of gene cluster sequences. In this study, a sp. strain X20 with high MC-degrading ability was isolated, and the gene activity was verified by heterologous expression. The whole sequence of the gene cluster in strain X20 was obtained through PCR and thermal asymmetric interlaced (TAIL)-PCR, and then used for evolutionary origin analyses together with the sequences available in GenBank. Phylogenetic analyses of gene clusters suggested that the four genes had the same origin and evolutionary history. Genomic island analyses showed that there is a genomic island on the genome of sphingomonads that is capable of degrading MCs, on which the gene cluster anchors. The concentrated distribution of the gene cluster in sphingomonads implied that these genes have likely been present in the sphingomonads gene pool for a considerable time. Therefore, the gene cluster may have initially entered into the genome of sphingomonads together with the genomic island by a horizontal gene transfer event, and then become inherited by some sphingomonads. The species other than sphingomonads have likely acquired genes from sphingomonads by recently horizontal gene transfer due to the sporadic distribution of MC-degrading species and the genes in them. Our results shed new light on the evolutionary origin of the cluster and thus facilitate the interpretation of characteristic distribution of the gene in bacteria and the understanding of whole pathway.