The floral organs and wounded leaves of Nicotiana alata, the ornamental tobacco, produce a serine protease precursor molecule (NaPI) which is cleaved into six individual inhibitors of 6 kDa. Two of these inhibitors have chymotrypsin activity and four have trypsin activity. These proteinase inhibitors inhibit trypsins and chymotrypsins in the gut of Lepidopteran

pests, and when incorporated into artificial diets or transgenic plants they have a detrimental effect on growth and development of Hellcoverpa punctigera larvae, but some larvae are unaffected. This study describes the characterisation of one of the major targets for NaPI, the chymotrypsins and their potential role in the tolerance of larvae to ingestion of Napl. Chymotrypsin CDNAs were isolated from a CDNA library made from the gut H. punctigera larvae. Phylogenetic analysis of the encoded protein sequences indicated there were six major families of chymotrypsin genes in Lepidoptera.