A TOBACCO GENE ENCODING A NOVEL BASIC CLASS-II CHITINASE - A PUTATIVEANCESTOR OF BASIC CLASS-I AND ACIDIC CLASS-II CHITINASE GENES

Various chitinases have been identified in plants and categorized into several groups based on the analysis of their sequences and domains. We have isolated a tobacco gene that encodes a predicted polypeptide c onsisting of a 20-amino acid N-terminal signal peptide, followed by a 245-amino acid chitinolytic domain. Although the predicted mature prot ein is basic and shows greater sequence identity to basic class I chit inases (75%) than to acidic class II chitinases (67%), it lacks the N- terminal cysteine-rich domain and the C-terminal vacuolar targeting si gnal that is diagnostic for class I chitinases. Therefore, this gene a ppears to encode a novel, basic, class II chitinase, which we have des ignated NtChia2;B1. Accumulation of Chia2;B1 mRNA was induced in leave s in association with the local-lesion response to tobacco mosaic viru s (TMV) infection, and in response to treatment with salicylic acid, b ut was only slightly induced by treatment with ethephon. Little or no Chia2;B1 mRNA was detected in roots, flowers. and eel-suspension cultu res, in which class I chitinase mRNAs accumulate to high concentration s. Sequence comparisons of Chia2;B1 with known tobacco class I and cla ss II chitinase genes suggest that Chia2;B1 might encode an ancestral prototype of the present-day class I and class II isoforms. Possible m echanisms for chitinase gene evolution are discussed.