SEQUENCE VARIATION, DIFFERENTIAL EXPRESSION AND CHROMOSOMAL LOCATION OF RICE CHITINASE GENES

Rice chitinases are encoded by a small multigene family. To clarify th e overall organization of rice chitinase genes, we have isolated and c haracterized the genes Cht-1, Cht-2 and Cht-3. Although all the three genes encode class I chitinase, the nucleotide sequences of the coding regions of Cht-1 and Cht-3 are very similar (90%), while that of Cht- 2 is clearly more divergent (78%). Only Cht-2 has a 130 bp intron and encodes a C-terminal peptide sequence similar to that known to functio n as a vacuolar targeting signal. In 5' flanking regions of Cht-1 and Cht-3, but not of Cht-2, conserved sequences (GGCCGGCYGCCCYAG) were fo und. Related sequences were found also in the 5' flanking regions of a nother chitinase gene and a beta-glucanase gene which has also been re ported to be stress-induced in rice. RNA blot hybridization analysis d emonstrated that the stress-induced expression patterns of the Cht-1 a nd Cht-3 genes are similar, but quite different from that of Cht-2. Ho wever, all three genes are active in unstressed roots. By restriction fragment length polymorphism (RFLP) linkage analysis, Cht-1 and Cht-3 were mapped onto chromosome 6 and shown to be closely linked (0.8 cM). Cht-2 was mapped onto chromosome 5. All these features suggest that t he expression patterns of rice class I chitinase genes may be correlat ed with their levels of sequence divergence and their chromosomal loca tion.