The Aspergillus nidulans metR gene encodes a bZIP protein which activates transcription of sulphur metabolism genes

Abstract

The identification, isolation and characterization of a new Aspergillus nidulans positive-acting gene metR, which encodes a transcriptional activator of sulphur metabolism, is reported. metR mutants are tight auxotrophs requiring methionine or homocysteine for growth. Mutations in the metR gene are epistatic to mutations in the negative-acting sulphur regulatory scon genes. The metR coding sequence is interrupted by a single intron of 492 bp which is unusually long for fungi. Aspergillus nidulans METR is a member of bZIP family of DNA-binding proteins. The bZIP domains of METR and the Neurospora crassa CYS3 transcriptional activator of sulphur genes are highly similar. Although Neurospora cys-3 gene does not substitute for the metR function, a chimeric metR gene with a cys-3 bZIP domain is able to transform the AmetR mutant to methionine prototrophy. This indicates that METR recognizes the same regulatory sequence as CYS3. The metR gene is not essential, as deletion mutants are viable and have similar phenotype as point mutants. In contrast to the Neurospora cys-3, transcription of the metR gene was found to be regulated neither by METR protein nor by sulphur source. Transcription of metR gene is derepressed in the sconB2 mutant. Transcription of genes encoding sulphate permease, homocysteine synthase, cysteine synthase, ATP-sulphurylase, and sulphur controller - sconB is strongly regulated by the metR gene product and depends on the character of the metR mutation and sulphur supplementation.