Homology Between Chitinases That Are Induced by TMV Infection of Tobacco

Recently, four chitinases have been detected in tobacco mosaic virus (TMV) infected tobacco: two acidic chitinases that were identified as pathogenesis-related (PR) proteins P and Q and two basic chitinases (Legrand et al., Proc.Natl. Acad. Sci. USA, in press). Here, it was shown that P and Q are closely serologically related but not related to other known acidic tobacco PR proteins. Antisera to P and Q were used to characterize translation products of TMV-induced mRNAs that were hybrid-selected with cDNA clones described previously (Hooft van Huijsduijnen et al., EMBO J 5: 2057-2061, 1986). In this way cDNA clones corresponding to the acidic and basic chitinases were identified. The partial amino acid sequences of the acidic and basic tobacco chitinases that were represented in the clones, showed an approximately 70% homology to each other and to the sequence of a bean chitinase. Although the acidic and basic chitinases differ in apparent molecular weight, they were found to have homologous C-termini.Hybridization of cDNA probes to genomic blots indicated that the acidic and basic chitinases are each encoded by two to four genes in the amphidiploid genome of Samsun NN tobacco. A similar complexity was found for the genes encoding the tobacco PR protein that is homologous to the sweet-tasting protein thaumatin and to the bifunctional trypsin/α-amylase inhibitor from maize.