Isolation and characterization of cDNA clones encoding pathogenesis-related proteins from tobacco mosaic virus infected tobacco plants.

Infection of the tobacco cultivar Samsun NN by tobacco mosaic virus (TMV) results in a hypersensitive response. During this defense reaction several host encoded proteins, known as pathogenesis-related proteins (PR-proteins), are induced. Poly(A)+ RNA from TMV infected tobacco plants was used to construct a cDNA library. Thirty two cDNA clones were isolated and after digestion with different restriction endonucleases, twenty clones were found to code for PR-1a, six clones for PR-1b, and four clones for PR-1c. Two independent cDNA clones of each class were further characterized by DNA sequence analysis. All clones analyzed contained the 138 amino acid coding regions of their respective mature proteins, but only partial sequences of the signal peptides. Minor differences between the nucleotide sequences for clones belonging to the same class were detected. Comparison of the amino acid sequence for PR-1a deduced from its nucleotide sequence with published data obtained by Edman degradation of the protein showed four differences. Analysis of the 3' ends of the cDNA clones indicates that various alternate poly(dA) addition sites are used. Southern blot analysis using these cDNAs as probes suggests the presence of multiple PR-protein genes in the genomes of tobacco and tomato plants.