PARTIAL CLONING OF P45 FROM RESISTANT MELON REVEALS HIGH HOMOLOGY WITH AMINOTRANSFERASES

Cucumis melo PI 124111F carries the genes Pc-1 and Pc-2 for resistance against downy mildew (Pseudoperonospora cubensis). Previous studies showed that a constitutive, cytoplasmic, soluble 45 Kd protein, P45, cosegregates in PI 124111F with resistance to this disease. In the present study, P45 was purified and partially sequenced. For six partial sequences (7–26 amino acids in each) of P45, twenty degenerated oligonucleotide primers were synthesized and subjected to PCR. Two DNA products, 500 bp and 1035 bp were obtained. These DNA products were cloned, sequenced and subjected to RACE reactions. A 1428 bp DNA sequence was obtained which encodes to a 476 amino-acid protein. This protein was found to carry three out of the six sequences originally obtained from P45. Analysis with the BLAST program revealed over 80% homology of the N-terminus of this protein with serine-glycoxalate aminotransferase from Fritillaria agrestis and alanine-glyoxalate aminotransferase from Arabidopsis thaliana. We propose that resistance in PI 124111F is associated with unique expression and/or function of aminotransferases.