ANALYSIS OF TOX5 GENE-EXPRESSION IN GIBBERELLA-PULICARIS STRAINS WITHDIFFERENT TRICHOTHECENE PRODUCTION PHENOTYPES

The Tox5 gene encodes trichodiene synthase, the first unique enzyme in the trichothecene biosynthetic pathway. In Gibberella pulicaris R-638 0, the level of Tox5 mRNA was found to increase 47-fold in early stati onary phase. Sequence analysis of the Tox5 promoter regions from geogr aphically distinct strains of G. pulicaris revealed the existence of t wo Tox5 alleles (Tox5-1 and Tox5-2). All G. pulicaris strains that pro duce high levels of trichothecenes in liquid culture carry a 42-nucleo tide (nt) tandem repeat sequence (Tox5-1) in the Tox5 promoter region, whereas strains that produce low levels of trichothecenes carry a sin gle copy of this sequence (Tox5-2). A genetic cross between high- and low-level trichothecene producers resulted in the cosegregation of hig her-level trichothecene production with the Tox5-1 allele. To determin e the importance of the 42-nt repeat sequence in the regulation of Tox 5 expression, reporter gene constructs carrying either the Tox5-1 or t he Tox5-2 promoter region fused to the beta-galactosidase gene of Esch erichia coli were introduced into the high-level-trichothecene-produci ng strain, R-6380. Expression of reporter gene activity in transforman ts was found to be regulated in a manner similar to Tox5 expression bu t appeared to be independent of the 42-nt sequence copy number. These results indicate that transcriptional controls play an important role in the regulation of Tox5 expression and that genes involved in tricho thecene biosynthesis in G. pulicaris may be linked to Tox5.