A FATTY-ACID SYNTHASE GENE IN COCHLIOBOLUS-CARBONUM REQUIRED FOR PRODUCTION OF HC-TOXIN, LANYL-D-ALANYL-L-2-AMINO-9,10-EPOXI-8-OXODECANOYL)

The fungal maize pathogen Cochliobolus carbonum produces a phytotoxic and cytostatic cyclic peptide, HC-toxin, of structure cyclo(D-prolyl-L -alanyl-D-alanyl-L-Aeo), in which Aeo stands for 2-amino-9,10-epoxi-8- oxodecanoic acid, Here we report the isolation of a gene, TOXC, that i s present only in HC-toxin-producing (Tox2(+)) fungal strains, TOXC is present in most Tox2(+) strains in three functional copies, all of wh ich are on the same chromosome as the gene encoding HC-toxin synthetas e, When all copies of TOXC are mutated by targeted gene disruption, th e fungus grows and sporulates normally in vitro but no longer makes HC -toxin and is not pathogenic, indicating that TOXC has a specific role in HC-toxin production and hence virulence. The TOXC mRNA is 6.5 kb a nd the predicted product has 2,080 amino acids and a molecular weight of 233,000. The primary amino acid sequence is highly similar (45 to 4 7% identity) to the beta subunit of fatty acid synthase from several l ower eukaryotes, and contains, in the same order as in other beta subu nits, domains predicted to encode acetyl transferase, enoyl reductase, dehydratase, and malonyl-palmityl transferase, The most plausible fun ction of TOXC is to contribute to the synthesis of the decanoic acid b ackbone of Aeo.