The α-helix is a common element of secondary structure in proteins and peptides. In eukaryotic organisms, which exclusively incorporate L-amino acids into such molecules, stereochemical interactions make such α-helices, invariably right-handed. Pseudomonas tolaasii Paine is the causal organism of the economically significant brown blotch disease of the cultivated mushroom Agaricus bisporus (Lange) Imbach. P. tolaasii proceduces an extracellular lipodepsipeptide toxin, tolaasin, which causes the brown pitted lesions on the mushroom cap. Circular dichroism studies tolaasin in a membrane-like environment indicate the presence of a left-handed α-helix, probably formed by a sequence of 7 D-amino acids in the peptide. P. tolaasii represents the first reported example of an organism which has evolved the ability to biosynthesize a left-handed α-helix.
