Toxic Lichens

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Science  20 Apr 2012:
Vol. 336, Issue 6079, pp. 277
DOI: 10.1126/science.336.6079.277-d

Fungi and green algae can form symbioses called lichens—several involve cyanobacteria as the photosynthetic accessory. Nostoc, a filamentous cyanobacterium, is a common partner forming what are known as cyanolichens. Unfortunately, Nostoc, in common with fresh water bloom–forming cyanobacteria, can produce cyclic peptide toxins such as microcystins and nodularins, which target mammalian phosphatases and may be carcinogens. Kaasalainen et al. have discovered that many cyanolichens contain these toxins too, and have detected over 50 chemical variants of microcystins. The signature gene is mcyE, whose product is required for the synthesis of a variant amino acid; the bond between this amino acid and D-glutamate is essential for microcystin toxicity. But mcyE does not seem to be affected by the horizontal gene transfer that is rife among the cyanobacteria, and this amino acid is a constant in the otherwise variable peptide sequence of the toxin. So lichens with identical mcyE can have very different microcystin compositions. Being trapped in a relationship with a fungus tends to lead to local population bottlenecks, which may have been instrumental in the evolution of this surprising diversity of toxins.

Proc. Natl. Acad. Sci. U.S.A. 109, 10.1073/pnas.1200279109 (2012).

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