Active and silent members in the mlr gene cluster of a microcystin-degrading bacterium isolated from Lake Taihu, China

The microcystin-degrading genes, mlr, are important participants in the degradation process of hepatotoxic microcystins for several bacterial species. However, their expression status during degrading microcystins is still unknown. In order to study this expression process, we isolated a novel microcystin-degrading bacterial strain, sequenced its mlr gene cluster and examined the expression of the mlrA gene at different concentrations of microcystin LR. The expression of mlrA increased slightly at 0.4 mg L⁻¹, and was significantly upregulated at 2.0 mg L⁻¹. Frameshift mutations were found in the mlrB* gene, and the mRNA of mlrB* could not be detected in the total RNA extracts of Novosphingobium sp. THN1. We conclude that mlrA is actively involved in the microcystin-degrading process, but mlrB* has lost its activity in this bacterial strain.