Nontoxic strains of cyanobacteria are the result of major gene deletion events induced by a transposable element

Abstract

Blooms that are formed by cyanobacteria consist of toxic and nontoxic strains. The mechanisms that result in the occurrence of nontoxic strains are enigmatic. All the nontoxic strains of the filamentous cyanobacterium Planktothrix that were isolated from 9 European countries were found to have lost 90% of a large microcystin synthetase (mcy) gene cluster that encoded the synthesis of the toxic peptide microcystin (MC). Those strains still contain the flanking regions of the mcy gene cluster along with remnants of the transposable elements that are found in between. The majority of the strains still contain a gene coding for a distinct thioesterase type II (mcyT), which is putatively involved in MC synthesis. The insertional inactivation of mcyT in an MC-producing strain resulted in the reduction of MC synthesis by 94 +/- 2% (1 standard deviation). Nontoxic strains that occur in shallow lakes throughout Europe form a monophyletic lineage. A second lineage consists of strains that contain the mcy gene cluster but differ in their photosynthetic pigment composition, which is due to the occurrence of strains that contain phycocyanin or large amounts of phycoerythrin in addition to phycocyanin. Strains containing phycoerythrin typically occur in deep-stratified lakes. The rare occurrence of gene cluster deletion, paired with the evolutionary diversification of the lineages of strains that lost or still contain the mcy gene cluster, needs to be invoked in order to explain the absence or dominance of toxic cyanobacteria in various habitats.

FIG. 1.—

Origin of the strains of the cyanobacterium Planktothrix spp. isolated from European freshwater sites. Red triangles, strains containing the mcy gene cluster encoding the synthesis of the toxic heptapeptide MC (37 strains) and black dots, nontoxic strains lacking the mcy gene cluster (25 strains).

FIG. 2.—

Growth rate, MC content, and mcy transcripts of the wild-type Planktothrix strain CYA126/8 and its ΔmcyT mutant grown under semicontinuous culture conditions. (A) Mean (±1 SD) MC content (per mm³ of biovolume, black circles) and growth rates (μ d⁻¹, white circles). (B) The mRNA contents of mcyB in proportion to PC-IGS (in percentage of mcyB) for the same experiment.

FIG. 3.—

Schematic view of the mcy operon remnants and flanking regions in strains that lost the mcy gene cluster. The 4 types (I–IV) of gene cluster deletion events are shown. Vertical straight lines enclose the identical 5′ and 3′ ends. The gray gene regions represent the remnants of the IS elements (197 bp) containing terminal inverted repeats (29 bp or 48 bp) (black boxes). The dotted areas indicate the deletions. Red triangle, strains containing the mcy gene cluster and black dots, nontoxic strains lacking the mcy gene cluster.

FIG. 4.—

Maximum likelihood tree of Planktothrix strains containing mcyT as part of the mcy gene cluster (red triangles) and as a remnant of the mcy gene cluster (black dots). The numbers at the nodes indicate the percent bootstrap frequency (100 replicates) that was obtained from maximum likelihood–Neighbor-Joining–maximum parsimony calculated using the PHYLIP package. The tree was rooted using the Nostoc sp. strain PCC7120 as an outgroup. Only the bootstrap values of >50% are shown. Asterisks indicate strains that were inactivated by transposable elements but still contained the whole mcy gene cluster (Christiansen et al. 2006).

FIG. 5.—

MLST diagrams showing 2 clonal complexes of strains that lost (lineage 1) or still contained the mcy gene cluster (lineages 1 and 2). Red triangles and bold letters: strains containing the mcy gene cluster encoding the synthesis of the toxic heptapeptide MC (37 strains) and black dots and normal letters: nontoxic strains that lost the mcy gene cluster (25 strains). Lineage 1, central founder (11 strains), bootstrap confidence 85% and lineage 2, central founder (17 strains), bootstrap confidence 92%. The connected STs differ at only 1 sequenced locus. The size of the symbols is proportional to the number of strains representing each ST.