Although there are six genes encoding PSKs in Arabidopsis, the mature peptides (YIYTQ) encoded by each gene are identical (with the exception of AtPSK6, YIYTH). Presumably, the peptides encoded by each gene should be able to bind and activate the single known receptor, AtPSKR1 . Each of the AtPSK precursors has the typical subtilase recognition site signature (di-basic amino acids) 8-10 residues upstream from the mature peptide (Barr, 1991) . However, the residues that are critical for AtSBT1.1 recognition (the four or five residues just downstream from the di-basic site) differ somewhat between AtPSK genes, and therefore AtSBT1.1 appears to be most specific for cleavage of PSK4. That suggests that other subtilases might be involved in the processing of other PSKs.