A cDNA from tobacco codes for an inhibitor of virus replication (IVR)-like protein.

We have shown previously that localization of tobacco mosaic virus (TMV) in tobacco is associated with a ca. 23 kDa protein that inhibits replication of several plant viruses. This protein, named 'inhibitor of virus replication' (IVR), was purified from the medium of TMV-inoculated protoplasts derived from Nicotiana tabacum cv. Samsun NN. IVR was shown to be present also in induced-resistant leaf tissue of N. tabacum cv. Samsun NN. We prepared an expression cDNA library from such induced-resistant tissue and screened it with a polyclonal antibody raised against the IVR protein. A 1016 bp clone (named NC330) containing a 597 bp open reading frame, coding for a 21.6 kDa polypeptide, was isolated. The NC330 clone hybridized with RNA from induced-resistant tissue from N. tabacum cv. Samsun NN but not with RNA from non-induced tissue. Likewise, it did not hybridize with RNA from infected or uninfected tissue of N. tabacum cv. Samsun nn. Similarly, the NC330 cloned probe hybridized with the RT-PCR products from RNA of the induced-resistant tissue only. In Southern blot hybridization the NC330 DNA probe detected several genomic DNA fragments in both N. tabacum cv. Samsun NN and Samsun nn. The size of the DNA fragments differed in Samsun NN and Samsun nn. We suggest that DNA encoding the IVR-like protein is present in resistant and susceptible N. tabacum genotypes, but is expressed only in NN. We have inserted the NC330 into the expression vector pET22b and a 21.6 kDa protein was produced in Escherichia coli that reacted in immunoblots with the IVR antibody. This protein greatly reduced replication of TMV in N. tabacum cv. Samsun nn leaf disk assays.