Abstract
The majority of cloned resistance (R) genes characterized so far contain a nucleotide-binding site (NBS) and a leucine-rich repeat (LRR) domain, where highly conserved motifs are found. Resistance genes analogs (RGAs) are genetic markers obtained by a PCR-based strategy using degenerated oligonucleotide primers drawn from these highly conserved "motifs". This strategy has the advantage of the high degree of structural and amino acid sequence conservation that is observed in R genes. The objective of the present study was to search for RGAs in Carica papaya L. and Vasconcellea cauliflora Jacq. A. DC. Out of three combinations of primers tested, only one resulted in amplification. The amplified product was cloned in pCR2.1TOPO and than sequenced using M13 forward and reverse primers. Forty-eight clones were sequenced from each species. The 96 sequences generated for each species were cleaned of vector sequences and clustered using CAP3 assembler. From the GENEBANK, one RGA was identified in C. papaya showing a BlastX e-value of 2x10-61 to the gb|AAP45165.1| putative disease resistant protein RGA3 (Solanum bulbocastanum). To the extent of our knowledge this is the first report of a RGA in the Caricaceae Dumort family. Preliminary structural studies were performed to further characterize this putative NBS-LRR type protein. Efforts to search for other RGAs in papaya should continue, mostly to provide basis for the development of transgenic papaya with resistance to diseases.
Highlights
Carica papaya L. is the most well-known and cultivated species in the Caricaceae Dumort family
Multiple alignment of the C. papaya sequence with retrieved Resistance genes analogs (RGAs) demonstrates that all of them share the same conserved sequences described in other plant resistance genes (Pan et al, 2000)
The presence of two conserved phenylalanine residues, separated by four amino acids, comprises the kinase 2 motif, which is a characteristic feature of nucleotide-binding site (NBS)-leucine-rich repeat (LRR) proteins (Mago et al, 2002)
Summary
Carica papaya L. is the most well-known and cultivated species in the Caricaceae Dumort family. The Carica genus contains only one species, C. papaya, while the genera Vasconcellea contains several species, which were originally classified as belonging to the Carica genus, including Vasconcellea cauliflora Jacq. This species, known as “tapaculo”, “papayo de montaña” or “zonzapote” is found from the South of Mexico to the North of South America, as well as in Trinidad. Such species is a well-known source of natural resistance to Papaya Ringspot Virus (PRSV), the main virus attacking papaya worldwide (Badillo, 1993, 2000 e 2001; Manshardt & Wenslaff, 1989)
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