Biodegradation mechanism of microcystin-LR by a novel isolate of Rhizobium sp. TH and the evolutionary origin of the mlrA gene

• The frequent presence of microcystin (MC) in eutrophic water bodies worldwide poses a serious threat to ecosystems. Biodegradation has been extensively investigated as a main pathway for MC attenuation, and an mlr-dependent mechanism of MC degradation have been elucidated in detail. However, the evolutionary origin and the distribution of mlr genes in MC-degrading bacteria is poorly understood. In this study, a novel Rhizobium sp. TH, which is the first α-proteobacterial MC-degrading bacterium other than Sphingomonadales, was isolated. Strain TH degraded MC via the mlr-dependent mechanism with a first-order rate constant of 0.18–0.29 h−1 under near-natural conditions. The partial length mlr gene cluster was sequenced, and the function of its key gene, mlrA, was verified by heterologous expression in Escherichia coli. Phylogenetic analyses show that the mlrA gene initially arose in α-proteobacteria by vertical evolution, and the two strains from β- and γ-proteobacteria acquired it by horizontal gene transfer. Therefore, the mlrA gene mainly exists in α-proteobacteria but is seldom present in other bacteria. A pair of primers matching well with mlrA sequences reported so far were designed and could be used to determine the MC-degrading mechanism for novel isolates or to screen for MC-degrading ability among environmental samples.

Subject headings

NATURVETENSKAP  -- Biologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences (hsv//eng)

Keyword

Degradation

Evolutionary origin

Mechanism

Microcystin

mlrA gene

Rhizobium

Bacteria

Escherichia coli

Gene expression

Gene transfer

Genes

Mechanisms

Rate constants

Strain

Toxic materials

Environmental sample

First-order rate constants

Heterologous expression

Horizontal gene transfer

Microcystins

Phylogenetic analysis

Biodegradation

eutrophic environment

evolutionary biology

gene

phylogenetics

reaction kinetics

rhizobacterium

toxin

Alphaproteobacteria

Bacteria (microorganisms)

Gammaproteobacteria

Proteobacteria

Rhizobium sp.

Sphingomonadales

Publication and Content Type

ref (subject category)

art (subject category)