| Summary: | The <i>mlr</i>-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 <i>mlr </i>gene<i> </i>cluster sequences. In this study, a <i>Sphingopyxis</i> sp. strain X20 with high MC-degrading ability was isolated, and the<i> mlrA</i> gene activity was verified by heterologous expression. The whole sequence of the <i>mlr</i> 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 <i>mlr</i> gene clusters suggested that the four <i>mlr</i> 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 <i>mlr</i> gene cluster anchors. The concentrated distribution of the <i>mlr</i> gene cluster in sphingomonads implied that these genes have likely been present in the sphingomonads gene pool for a considerable time. Therefore, the <i>mlr</i> 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 <i>mlr</i> genes from sphingomonads by recently horizontal gene transfer due to the sporadic distribution of MC-degrading species and the <i>mlr</i> genes in them. Our results shed new light on the evolutionary origin of the <i>mlr</i> cluster and thus facilitate the interpretation of characteristic distribution of the <i>mlr</i> gene in bacteria and the understanding of whole <i>mlr</i> pathway.
|
|---|