We compared the sequence of AtPSK4 to some of the most closely related AtPSKs and to PSK-related sequences in other plants. The five amino acids representing the presumed mature PSK peptide in the six Arabidopsis sequences are nearly identical (Table 1) other plant PSKs in the GenBank database, except for AtPSK6. The upstream di-basic amino acids and the leucine (P3 position) and histidine and aspartate (P1¢ and P3¢, respectively) are also conserved in Arabidopsis. To compare the activity of AtSBT1.1 with other AtPSK precursors, we developed fluorogenic peptides representing putative recognition sites for the family of AtPSKs (fpAtPSK1, 2, 3, 5 and 6, see bold residues in Table 1 ). The putative recognition sites for pAtPSK2 and 5 are the same and are represented in the fluorogenic peptides fpPSK2 and 5. The cleavage reaction was slower for fpAtPSK2 and 5 than for fpAtPSK4, and was barely detectable or not detectable at all with the other fpAtPSKs ( Figure 5b) . Thus, the activity of AtSBT1.1 is fairly specific, with most activity directed toward cleavage of pAtPSK4, followed by pAtPSK2 and 5.