The golden alga Prymnesium parvum produces a complex toxin that causes large-scale fish deaths during algal blooms in various parts of the world. Researchers have now discovered how the alga produces this poison. They say it produces huge enzymes that assemble the complex poison molecules. One of these enzymes is the largest protein discovered in nature to date. The discovery can help to better monitor harmful algal blooms. It could also open up new strategies for drug development.
The single-celled alga Prymnesium parvum is widespread worldwide and occurs mainly in salt and brackish water. Under certain conditions, extensive algal blooms occur, which often lead to mass deaths of fish. The reason for this is that the single-celled algae release toxins under stress, so-called prymnesin compounds. These probably actually serve to make it easier for the algae to hunt other microorganisms and to ward off predators. In larger quantities, as can occur in an algal bloom, the poison also has a neurotoxic effect on fish and sometimes causes great damage to fish cultures and in the wild. In the summer of 2022, this algae caused a dramatic mass death of fish in the Oder.
The algae toxins of Prymnesium parvum are extremely complex and consist of numerous chained ring compounds at the molecular level. “The biosynthetic origin of these compounds has been a mystery to science for more than 40 years,” writes a team led by Timothy Fallon from the University of California in San Diego. To solve this mystery, the researchers first sequenced the entire genome of P. parvum and then looked for the genes involved in the production of the toxin.
Giant enzymes
“We were able to locate the genes, and it turned out that this algae uses huge genes to produce huge toxic molecules,” reports Fallon’s colleague Vikram Shende. The researchers read the amino acid sequence of the encoded proteins from the genetic code and reconstructed them. They discovered that these were so-called polyketide synthases (PKS), i.e. enzymes that produce polyketides such as the algae toxin. Due to their extraordinary size, Fallon and his team gave them the names PKZILLA-1 and PKZILLA-2.
Further analysis of the enzymes revealed their record-breaking mass: PKZILLA-1 weighs 4.7 megadaltons – more than a hundred times as much as an average protein. The largest natural protein known to date, the structural protein titin, which is found in human muscle cells, among other things, weighs 3.7 megadaltons. PKZILLA-2 weighs 3.2 megadaltons. “This expands our idea of what biology is capable of,” says Fallon’s colleague Bradley Moore. PKZILLA-1 contains 140 domains where enzymatic reactions take place, PKZILLA-2 contributes 99 enzyme domains. As the research team showed, the two enzymes catalyze a sequence of 239 chemical reactions that ultimately produce the complex prymnesin compounds.
Detect harmful algal blooms earlier
“The hope is that we can use this knowledge of how nature produces these complex chemicals to explore new chemical possibilities in the laboratory for the medicines and materials of tomorrow,” says Moore. Various medicines, including several antibiotics and chemotherapeutic agents, have similar structural features to the algal toxin. In addition, the new findings could also help to identify harmful algal blooms earlier. Previous tests aim to detect the toxins in the water – so they only detect them when it is actually already too late.
However, knowing the genes responsible makes it possible to search for them directly. “Monitoring the genes rather than the toxin could enable us to detect blooms before they occur, rather than only identifying them when the toxins are already in circulation,” says Fallon. In the future, the research team also wants to understand the synthesis pathways of other algal toxins and in this way make it possible to monitor other toxic algal blooms, some of which are also dangerous to humans, using genetic knowledge.
Source: Timothy Fallon (University of California, San Diego, La Jolla, USA) et al., Science, doi: 10.1126/science.ado3290