Abstract
The frequent production of the hepatotoxin microcystin (MC) and its impact on the lifestyle of bloom-forming cyanobacteria are poorly understood. Here, we report that MC interferes with the assembly and the subcellular localization of RubisCO, in Microcystis aeruginosa PCC7806. Immunofluorescence, electron microscopic and cellular fractionation studies revealed a pronounced heterogeneity in the subcellular localization of RubisCO. At high cell density, RubisCO particles are largely separate from carboxysomes in M. aeruginosa and relocate to the cytoplasmic membrane under high-light conditions. We hypothesize that the binding of MC to RubisCO promotes its membrane association and enables an extreme versatility of the enzyme. Steady-state levels of the RubisCO CO2 fixation product 3-phosphoglycerate are significantly higher in the MC-producing wild type. We also detected noticeable amounts of the RubisCO oxygenase reaction product secreted into the medium that may support the mutual interaction of M. aeruginosa with its heterotrophic microbial community.
Highlights
Bloom-forming cyanobacteria are infamous for the production of toxins and constitute a serious threat for humans and animals
To evaluate whether the light-dependent dynamics in the accumulation of RubisCO products in M. aeruginosa (Meissner et al, 2015) is due to changes in the subcellular localization of the enzyme, we performed light shift experiments for up to 4 h with cells of the MC-producing wild-type strain (WT) and the MC-deficient DmcyB mutant, which were pre-grown under low light conditions at ambient air
The overall amount of the metabolite was rather low and differences between wild type (WT) and mutant strain were neglectable. These data indicate that the ratio between carboxylation and oxygenation activity of RubisCO differs between the two strains, whereby the RubisCO carboxylase activity is favoured in the MC-producing WT
Summary
Bloom-forming cyanobacteria are infamous for the production of toxins and constitute a serious threat for humans and animals. Stands out as the most widely encountered type of toxin (Dittmann, Fewer, & Neilan, 2013; Huisman et al, 2018). The toxicity of the cyclic heptapeptide was attributed to the inhibition of eukaryotic-type protein phosphatases of the families PP1 and PP2A (Goldberg et al., 1995). A phylogenetic analysis of MC biosynthesis genes from distant cyanobacterial genera revealed that the genes were already present in the last common ancestor of all recent cyanobacteria and prior to the evolution of eukaryotes (Rantala et al, 2004). MCs are primarily intracellular toxins and the levels dissolved in water typically do not exceed critical levels of toxicity (Dittmann et al, 2013) While exposure to MC is often fatal for animals, a primary role of MC as feeding deterrent is increasingly under debate (Rohrlack, Dittmann, Borner, & Christoffersen, 2001).
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