The trichothecene biosynthesis involves multiple enzymatic reactions controlled by multiple genes. Biochemical and genetic studies of trichothecene pathway, initially in F. sporotrichioides using overlapping cosmid clones has resulted in establishing the consensus map of the trichothe-cene pathway gene cluster on a 25 kb region of the F. sporotrichioides chromosome (Brown et al. 2001; Hohn et al. 1993a; Proctor 2000; Trapp et al. 1998). This gene cluster consists of ten genes with the designation, in sequential order, TRI8, TRI7, TRI3, TRI4, TRI6, TRI5, TRI10, TRI9, TRI11, and TRI12, respectively (Figure 2, Brown et al. 2001). In F. graminearum, a similar gene cluster has been identified except that TRI7 in F. graminearum is nonfunctional (Brown et al. 2001; Matsumoto et al. 1999; Proctor et al. 1995a; 1997. The gene organization and direction of transcription are identical in both F. sporotrichioides and F. graminearum. In both F. sporotrichioides and F. graminearum, the eleventh gene involved in trichothecene biosynthesis, TRI101, was isolated outside the established trichothecene pathway gene cluster by Kimura et al. (1998a), McCormick et al. (1999). Wuchiyama et al. (2000) identified the TRI12 counterpart gene in F. graminearum designated, TRI102, which encodes a trichothecene efflux pump. Lee et al. (2001) identified a similar trichothecene gene cluster in Gibberella zeae in the order of, TRI8, TRI7, TRI3, TRI4, TRI6, TRI5, TRI10, TRI9, TRI11, and TRI102, respectively. The TRI102 in Gibberella zeae shares significant homology with the TRI12 and TRI102 in F. sporotrichioides and F. graminearum. However, the TRI102 in Gibberella zeae was found to be nonfunctional
Figure 2 Biochemical pathway and the specific genes and their enzymes involved in trichothecene biosynthesis in Fusarium species. Note that arrows in the gene cluster indicate the direction of gene transcription. (Figure courtesy Daren W. Brown, with modifications from Brown et al. 2001).
(Lee et al. 2001). Note that TRI1 and TRI2 were named based on UV-generated nontrichothecene-producing mutants. So far no corresponding gene or DNA sequences on molecular level have been identified yet and the location of these mutated loci are still unclear (Robert H. Proctor, personal communication). The functions of most of the identified genes have been proposed or characterized except TRI9 which encodes the smallest peptide consisting of only 43 amino acids with its function unknown (Figure 2, Brown et al. 2001).
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