Clones corresponding to tobacco pathogenesis-related (PR)
proteins PR-4 and tomato PR
protein P2 were isolated from phage cDNA libraries of tobacco infected with tobacco mosaic virus and tomato infected with Cladosporium fulvum, respectively. The probe used in these screenings was a polymerase chain reaction product, synthesized on phage
DNA from the tobacco cDNA library, using a synthetic
oligonucleotide primer whose sequence corresponded to the partial amino acid sequence available for P2. The different
cDNA sequences from the tobacco and tomato clones contained open reading frames for small
proteins with 80-90% amino acid sequence identity. Both tobacco PR-4 and tomato P2 are synthesized as precursor
proteins, with an N-terminal
signal peptide involved in extracellular targeting. The
proteins are highly similar to putative
wound-induced
proteins of potato (win) and to the precursor
protein of
hevein. However, in contrast to the
hevein pro-
protein and win
proteins, PR-4 and P2 do not contain N-terminal,
chitin-binding "
hevein" domains. The tobacco and tomato genomes contain a limited number of genes corresponding to PR-4 or P2, whose expression is induced upon
infection with the above-mentioned pathogens.