Alleles of Pto and Fen occur in bacterial speck-susceptible and fenthion-insensitive tomato cultivars and encode active protein kinases.

The Pto gene was derived originally from the wild tomato species Lycopersicon pimpinellifolium and confers resistance to Pseudomonas syringae pv tomato strains expressing the avirulence gene avrPto. The Fen gene is also derived from L. pimpinellifolium and confers sensitivity to the insecticide fenthion. We have now isolated and characterized the alleles of Pto and Fen from cultivated tomato, L. esculentum, and designated them pto and fen. High conservation of genome organization between the two tomato species allowed us to identify the pto and fen alleles from among the cluster of closely related Pto gene family members. The pto and fen alleles are transcribed and have uninterrupted open reading frames that code for predicted proteins that are 87 and 98% identical to the Pto and Fen protein kinases, respectively. In vitro autophosphorylation assays revealed that both the pto and fen alleles encode active kinases. In addition, the pto kinase phosphorylates a previously characterized substrate of Pto, the Pto-interacting Pti1 serine/threonine kinase. However, the pto kinase shows impaired interaction with Pti1 and with several previously isolated Pto-interacting proteins in the yeast two-hybrid system. The observation that pto and fen are active kinases and yet do not confer bacterial speck resistance or fenthion sensitivity suggests that the amino acid substitutions distinguishing them from Pto and Fen may interfere with recognition of the corresponding signal molecule or with protein-protein interactions involved in the Pto- and Fen-mediated signal transduction pathways.