The helicase domain of the TMV replicase proteins induces the N-mediated defence response in tobacco

Tobacco mosaic virus (TMV) induces the hypersensitive response (HR) in toba cco plants containing the N gene. This defence response is characterized by cell death at the site of virus infection and inhibition of viral replicat ion and movement. A previous study indicated that a portion of the IMV repl icase containing a putative helicase domain is involved in HR induction. He re, this observation is confirmed and extended by showing that non-viral ex pression of a 50 kDa TMV helicase fragment (p50) is sufficient to induce th e N-mediated HR in tobacco. Like the HR elicited by TMV infection, transgen ic expression of p50 induces a temperature-sensitive defence response. We d emonstrate that recombinant p50 protein has ATPase activity, as suggested b y the presence of conserved sequence motifs found in ATPase/helicase enzyme s. A point mutation that alters one of these motifs abolishes ATPase activi ty in vitro but does not affect HR induction. These results suggest that fe atures of the TMV helicase domain, independent of its enzymatic activity, a re recognized by N-containing tobacco to induce TMV resistance.