Sulfite reductase (SIR) represents a key enzyme in sulfate assimilation in higher plants. The genomic DNA sequence of the sir gene from Arabidopsis thaliana including regulatory and structural regions was isolated and characterized. The sequence of a 6 kb fragment encoding SIR revealed a coding region of 2891 basepairs (bp) that consists of eight exons separated by seven introns between 83 and 139 bp in length. The transcription start point was determined 272 bp upstream of the translation start site. Southern analysis indicates a single locus for the sir gene that gives rise to a 2.4 (kb) mRNA in leaves and in roots. The promoter region was verified by functional expression of the gusA reporter gene in transgenic A. thaliana plants and was shown to provide correct expression in root and leaf.