Isolation of T-DNA mutants in the SULTR1;3 and SULTR2;1 genes. (a) Gene structure of SULTR1;3 and SULTR2;1. Gray boxes represent promoter regions, black boxes represent exons, and lines represent introns (not drawn to scale). Insertion sites of T-DNA and orientation (from left to right borders) are represented by triangles with arrowheads. Positions (base pairs from the start codon), and sequences analogous to the PHR1-binding motif present in the promoter are indicated. (b) The absence of SULTR1;3 and SULTR2;1 transcripts in the sultr1;3 and sultr2;1 mutants, respectively, was confirmed by RT-PCR. The expression of tubulin (TUB) was used as a control.