FIG. 5.

Proposed metabolic grid for biosynthesis of the type A trichothecenes. The preferred T-2 toxin biosynthetic pathway (boldface arrows) has been previously established in F. sporotrichioides NRRL 3299 (7, 8, 9, 10, 28). Thin arrows, major alternative biosynthetic routes; dashed arrows, minor routes. Any one of the alternative routes can become the predominant route in response to limitations imposed by a variety of mechanisms, including mutations or other perturbations that alter substrate availability or inhibit the activity of one or more of the TRI enzymes. For example, the routes indicated by the thin arrows define the preferred alternative routes in response to single gene mutations that inactivate either Tri11 (1), Tri13 (10), or Tri7 (9). MAS, monoacetoxyscirpenol.