Abstract
Bacterial degradation of toxic microcystins produced by cyanobacteria is a common phenomenon. However, our understanding of the mechanisms of these processes is rudimentary. In this paper several novel discoveries regarding the action of the enzymes of the mlr cluster responsible for microcystin biodegradation are presented using recombinant proteins. In particular, the predicted active sites of the recombinant MlrB and MlrC were analyzed using functional enzymes and their inactive muteins. A new degradation intermediate, a hexapeptide derived from linearized microcystins by MlrC, was discovered. Furthermore, the involvement of MlrA and MlrB in further degradation of the hexapeptides was confirmed and a corrected biochemical pathway of microcystin biodegradation has been proposed.
Highlights
Cyanobacterial secondary metabolites including toxic microcystins (MCs) are subject to microbial, mainly bacterial, degradation in natural water supplies
Recombinant MC-degrading proteins may be very helpful in the investigation of the biochemical pathways and subsequent MC derivatives, and this approach has been applied in this study
Due to discrepancies of the MlrB sequence length with homologous proteins in other bacteria, as well as previous experiences with the correction of MlrA [9], we determined the sequence of the upstream mlrB using inverse PCR
Summary
Cyanobacterial secondary metabolites including toxic microcystins (MCs) are subject to microbial, mainly bacterial, degradation in natural water supplies. The mechanism of MC biodegradation is poorly known and the knowledge is limited to one partially-recognized biochemical pathway [2,3] which involves three enzymes (MlrA, MlrB, and MlrC) encoded by the mlr gene cluster. Sphingomonas sp., Sphingophyxis sp., and phylogenetically distinct, bacterial species were found to contain homologues of the mlrA gene encoding the enzyme responsible for MC linearization, a crucial step for toxin degradation [1]. [4], mlr genes were not detected. This could mean that investigated strains do not possess mlr homologues, but this hypothesis requires experimental verification. Regardless of the above, the mechanisms of MC-degradation in other bacterial strains may be different and not limited to the proteins of mlr clusters. The contribution of the cell wall-associated proteinases was Toxins 2016, 8, 76; doi:10.3390/toxins8030076 www.mdpi.com/journal/toxins
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