Having identified conditions for pAtPSK4-myc cleavage, we wished to determine whether AtSBT1.1 was responsible for the proteolysis. To do so, we examined cleavage of pAtPSK4-myc in T-DNA mutants with insertions in AtSBT1.1. The gene encoding AtSBT1.1 (At1g01900) has a single intron, and sbt1.1-1 (SALK_111561) and sbt1.1-2 (SALK_108704) have T-DNA insertions in the first exon. The T-DNA lines were judged to be null mutants because AtSBT1.1 transcripts were not found in seedlings from either line ( Figure S2 ). The 35S:ppAtPSK4-myc construct was introduced into the two T-DNA mutant lines, and root explants were subjected to regeneration conditions (4 days incubation on CIM). The pAtPSK4-myc precursor was produced in these lines, but the cleavage product AtPSK4-myc peptide was not detected (Figure 1a , lanes sbt1.1-1 and sbt1.1-2). To demonstrate that the transgenic product in these lines was still cleavable, we out-crossed sbt1.1-1 bearing the 35S:ppAtPSK4-myc construct to wildtype, and demonstrated that processing was restored in root explants of the F 1 seedlings ( Figure S3 ). We concluded from these results that AtSBT1.1 is required for pAtPSK4-myc cleavage under these conditions. This finding is quite significant given that there are 56 different subtilases encoded by the Arabidopsis genome (Rautengarten et al., 2005) .