RESISTANCE TO BENOMYL CONFERRED BY MUTATIONS IN CODON-198 OR CODON-200 OF THE BETA-TUBULIN GENE OF NEUROSPORA-CRASSA AND SENSITIVITY TO DIETHOFENCARB CONFERRED BY CODON-198

In previous work, we established that point mutations in codon 200 (200Phe to Tyr) or 198 (198Glu to Ala) of the beta-tubulin gene were associated with medium and very high resistance to benomyl in field strains of several plant-pathogenic fungi. Site-directed mutagenesis was performed to introduce identical mutations in codon 198 or 200 of a beta-tubulin gene from Neurospora crassa. At position 198 a codon for glutamic acid was altered to a codon for alanine. Alternatively, at position 200 a codon for phenylalanine was altered to a codon for tyrosine. Transformants were obtained that exhibited resistance to benomyl after transformation with plasmid constructs. Southern analysis confirmed that additional copies of beta-tubulin DNA were present in the transformants. Transformants with beta-tubulin DNA that were integrated with the codon for alanine also exhibited sensitivity to diethofencarb. Therefore, we conclude that mutations in codon 198 or 200 of the beta-tubulin gene confer resistance to benomyl in several benomyl-resistant plant-pathogenic fungi, as demonstrated in this study for N. crassa, and that an alanine codon at position 198 is associated with sensitivity to diethofencarb.