ORGANIZATION AND CHARACTERIZATION OF CUCURBITA PHLOEM LECTIN GENES

The phloem of pumpkin and squash contains a dimeric chitin-binding lec tin called PP2 (phloem protein 2). We have isolated three genomic clon es from pumpkin (Cucurbita maxima Duch.) that encoded PP2. One clone, lambda gPC13-1, contained two PP2 genes that were 99.8% identical over a region of 3055 nucleotides. This conserved region included 1922 bp of 5' non-coding sequence, 844 bp of protein coding sequence (includin g two introns), and 289 bp of 3' non-coding sequence. To examine the c onservation of the phloem lectin within the genus Cucurbita, we analyz ed nine different species for PP2, its mRNA, and the genes that encode PP2. DNA blot analysis indicated that each species contained genes th at encoded PP2, however, there was considerable restriction fragment l ength polymorphism (RFLP) among the species. PP2 gene copy number reco nstructions indicated that PP2 is encoded by a small gene family (two to eight genes). Although a high level of PP2 DNA polymorphism existed among species, a single mRNA (ca. 1 kb) was detected in each species. PP2, affinity-purified from the vascular exudate of each species, rea cted with PP2-specific antibodies; five species contained a single PP2 polypeptide while four species contained two PP2 polypeptides.