Department of Plant Pathology and Microbiology, Texas A&M University, College Station, TX, USA

Background and objectives
The trichothecene mycotoxins produced by Fusarium spp. serve as virulence factors in some plant-pathogen interactions [1]. Consequently we are interested in the genetics and regulation of the production of this group of fungal secondary metabolites. One important aspect toward this goal is the identification of the genes specifically required for trichothecene production and the delineation of their structural organization within the genome, particularly as it pertains to the regulation of their expression. One of the recognized novel features of the genes for the production of secondary metabolites in fungi is their organization in large gene clusters [2]. Such clusters have been identified for the trichothecene genes in Fusarium sporotrichioides and in Myrothecium roridum. The main focus of this report is the identification of the trichothecene gene cluster in Fusarium sambucinum and its comparison to the trichothecene gene cluster in F. sporotrichioides.

Results and conclusions
A trichothecene gene cluster has been identified in F. sambucinum. A region spanning over 30 kb has been cloned and sequenced, and both structural and regulatory homologues of trichothecene genes in F. sporotrichioides are present. Comparison of the corresponding cloned DNA regions from the Tri gene clusters in F. sambucinum and F. sporotrichioides indicates these two species share nearly 90% identity at the nucleotide level for protein-coding sequences, and about 95% identity at the amino-acid level. The relative gene order, orientation of transcription, and spacing of all the genes are also fairly well conserved between the analysed portions of the F. sporotrichioides and F. sambucinum Tri gene clusters. Most notable is the conserved organization of the two identified regulatory genes, Tri6 and Tri10, which flank Tri5, the gene encoding the first enzyme in the trichothecene biosynthetic pathway.

Overall, the maintenance of the genes for trichothecene production in clusters has been preserved among species as diverse as Fusarium spp. and Myrothecium roridum. However, there is some variability in the organization of the genes within these clusters. These similarities and differences suggest that there may be important relationships between the clustering, the particular organization of the clustered genes and the genetic mechanisms that regulate toxin production.

1. Desjardins AE, Proctor RH, Bai G et al., 1996. Molecular Plant-Microbe Interactions 9, 775-781.
2. Keller NP, Hohn TM, 1997. Fungal Genetics and Biology 21, 17-29.