We characterized cDNAs encoding putative S-like RNases, RNase Nk1, Nk2 and Nk3, in Nicotiana tabacum L. cv. ky 57 leaf and examined the relation between these genes and Cucumber mosaic virus (CMV) infection. Of these genes, only RNase Nk1 was induced by wounding or CMV inoculation. The deduced amino acid sequence of RNase Nk1 is 97% identical to the N. alata RNase NE that is induced in roots in response to phosphate limitation. RNase Nk1 mRNA was highly induced in CMV-inoculated leaves within 3-6 hr after inoculation whereas a smaller increase was observed in mock- or CMV RNA-inoculated leaves. RNase Nk1 mRNA was also highly induced in leaves inoculated with CMV coat protein (CP) or BSA, suggesting that not only a mechanical damage but CP of CMV is the causal agent in the higher induction of RNase Nk1 gene immediately after virus infection in host plant cells
