Biosynthesis of Guanitoxin Enables Global Environmental Detection in Freshwater Cyanobacteria

Abstract

Harmful cyanobacterial blooms (cyanoHABs) cause recurrent toxic events in global watersheds. Although public health agencies monitor the causal toxins of most cyanoHABs and scientists in the field continue developing precise detection and prediction tools, the potent anticholinesterase neurotoxin, guanitoxin, is not presently environmentally monitored. This is largely due to its incompatibility with widely employed analytical methods and instability in the environment, despite guanitoxin being among the most lethal cyanotoxins. Here, we describe the guanitoxin biosynthesis gene cluster and its rigorously characterized nine-step metabolic pathway from L-arginine in the cyanobacterium Sphaerospermopsis torques-reginae ITEP-024. Through environmental sequencing data sets, guanitoxin (gnt) biosynthetic genes are repeatedly detected and expressed in municipal freshwater bodies that have undergone past toxic events. Knowledge of the genetic basis of guanitoxin biosynthesis now allows for environmental, biosynthetic gene monitoring to establish the global scope of this neurotoxic organophosphate.

Expression ofMicrocystisbiosynthetic gene clusters in natural populations suggests temporally dynamic synthesis of novel and known secondary metabolites in western Lake Erie

“…Other transcriptionally active BGCs encode unknown compounds with unknown toxicology. Given the continued emergence and discovery of novel cyanotoxins and their underpinning BGCs (Breinlinger et al ., 2021; Lima et al ., 2022), and the potential for toxicological synergism between multiple compounds (Fernandes et al ., 2019; Pawlik-Skowrońska and Bownik, 2022), these results underscore the need to assess potential threats to human and ecosystem health. In addition to potential risks of unmonitored toxic cyanobacteria secondary metabolites, these molecules may be an untapped source for drug discovery (Aesoy and Herfindal, 2022).…”

Transcriptionally active nitrogen fixation and biosynthesis of diverse secondary metabolites by Dolichospermum and Aphanizominom-like Cyanobacteria in western Lake Erie Microcystis blooms

“…appear to be rarer now in the open waters of the Great Lakes but can dominate blooms in tributaries (McKay et al, 2020) despite dominating Lake Erie blooms in the 1960s and 1970s (Davis, 1964; Munawar and Munawar, 2011). This dynamic distribution of ADA organisms in time and space, taken together with evidence of potential for biosynthesis of saxitoxin (Chaffin et al, 2019) and guanitoxin (Lima et al, 2022) in the Great Lakes, underscore the need to determine environmental and ecological controls on their abundance.…”

“…taken together with evidence of potential for biosynthesis of saxitoxin (Chaffin et al, 2019) and guanitoxin (Lima et al, 2022) in the Great Lakes, underscore the need to determine environmental and ecological controls on their abundance.…”

Genome Mining for Unknown-Unknown Natural Products

“…6 For example, a BGC that contains a cluster of tailoring enzymes and no canonical core enzyme is a reasonable starting point, as evidenced by recent characterization of bacterial BGCs of altemicidin, 6 fluopsin, 7 and guanitoxin. 8 These tailoring enzymes can include redox enzymes such as oxidoreductases, 9 transferring enzymes such as methyl-, acetyl-, or glycosyl-transferases; 10 and other enzymes that are found frequently in BGCs, such as PLP-dependent 11 or ATP-grasp enzymes. 12 In addition, the presence of ORFs that are designated as hypothetical proteins (HPs) or proteins predicted to have domains of unknown function (DUFs) is predictive of new enzymatic function.…”