A functional cDNA for the resistance gene N from Nicotiana tabacum cv. Samsun NN to Tobacco mosaic virus (TMV) was obtained by reverse transcription polymerase chain reaction. Recombinant proteins containing various domains of the N protein were used in far-Western screening for the N-interacting protein from tobacco cDNA expression libraries. Nucleotide sequence analysis revealed that 199 cDNAs of 217 positive clones screened from 1.6 × 106 phage plaques were related to the 14-3-3 gene family. Phylogenetic analysis indicated that the 14-3-3 proteins were divided into 17 different 14-3-3 isoforms, including hitherto unidentified novel isoforms i-1 and i-2. In vitro binding assays showed that the probes TIR and LRR domains of N protein and helicase (HEL) domain of TMV replicase bound significantly to 14-3-3 isoforms of groups ω and ψ. These domains barely bound to those of group κ and did not bind to that of group ε. The same probes did not bind to any domains of N protein or the viral HEL domain. Northern blotting showed an increase in 14-3-3 isoform h transcripts coinciding with PR-1a induction after TMV inoculation. The 14-3-3 isoform h as a probe had significant binding to the LRR domain of N protein and the viral HEL domain, but there was only minor binding to the TIR domain. These results raise the possibility that 14-3-3 acts as a scaffolding or adaptor between the N protein and the helicase domain to support the interaction between the R gene product and the viral elicitor.
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