A single amino-acid change in the iron-sulphur protein subunit of succinate dehydrogenase confers resistance to carboxin in Ustilago maydis.

The sequence of an allele encoding the iron-sulphur protein (Ip) subunit of succinate dehydrogenase (Sdh) was determined following PCR amplification of genomic DNA from a carboxin (Cbx)-sensitive Ustilago maydis strain. Comparison of this sequence with that of the Ip allele from a Cbx-resistant strain (IPr) revealed a two-base difference between the sequences. This mutation led to the substitution of a leucine residue for a histidine residue within the third cysteine-rich cluster of the deduced amino-acid sequence of the Ipr allele. This cluster, which is associated with the S3 iron-redox centre, is involved in the transport of electrons from succinate to ubiquinone (Q). Confirmation that this nucleotide change led to enhanced resistance to Cbx was obtained following mutagenesis of the sensitive Ip allele to the resistant form and expression of the mutated allele in U. maydis.