A single amino-acid substitution in the iron-sulphur protein subunit of succinate dehydrogenase determines resistance to carboxin in Mycosphaerella graminicola.

A gene encoding the iron-sulphur protein (Ip) subunit of succinate dehydrogenase (Sdh, EC 1.3.99.1) from Mycosphaerella graminicola (Septoria tritici) has been cloned andsequenced. The deduced amino-acid sequence exhibited a high degree of homology to Ip subunits of Sdh from other organisms; three cysteine-rich clusters associated with the iron-sulphur centres involved in electron transport were particularly conserved. Expression studies using a synthetic green fluorescent protein (SGFP) expression vector demonstrated that the cloned DNA also contained a functional promoter region and confirmed that the deduced initiation codon could act as a translational start site. Mutants resistant to the fungicide carboxin (Cbx), a known inhibitor of Sdh, were found to contain a single amino-acid substitution in the third cysteine-rich domain of the Ip protein. These mutations resulted in the conversion of a highly conserved His residue, located in a region of the protein associated with the [3Fe-4 S] high-potential non-heme iron sulphur-redox (S3) centre, to either Tyr or Leu. AnIp gene containing the His -> Tyr mutation was constructed and shown to confer Cbx resistance following co-transformation into the Cbx-sensitive wild-type strain. This confirmed that the mutation identified by sequence analysis was responsible for determining Cbx resistance.