植物几丁质酶的结构、基因及其表达

Primary Structure of Plant Chitinases and Genes Encoding Plant Chitinases

植物几丁质酶按其蛋白氨基酸序列结构特征及同源性可分为六类，即：ＣｌａｓｓＩ－Ⅵ。ＣｌａｓＩ在蛋白氨基酸结构上包括三个功能区域，Ｎ－端是富含半胱氨酸的几丁质结合区，约４０个氨基酸；Ｃ－端是酶的催化区，也是酶的主要功能区域，约３００个氨基酸；二者通过一个多变的交联区连接在一起。ＣｌａｓｓⅡ仅具有类似于ＣｌａｓｓⅠ的酶催化区域，而没有几丁质结合区和交联区。ＣｌａｓｓⅢ几丁质酶在氨基酸序列上与ＣｌａｓｓⅠ和Ⅱ没有任何同源性，其中有些具有几丁质酶和溶菌酶双重活性。ＣｌａｓｓⅣ类似于ＣｌａｓｓⅠ，只是在几丁质结合区和催化区缺失了少数氨基酸。ＣｌａｓｓＶ类似于ＣｌａｓｓⅠ，但具有两个重复的几丁质结合区。ＣｌａｓＶＩ与前五类几丁质酶无同源性，但与微生物几丁质酶有同源性。所有的植物几丁质酶都是由一个小的多基因族编码的，一般基因中有二个内含子，都位于催化区内。几丁质酶的表达受病原物和植物激素的诱导而表达，也与植物的发育有关。通过转几丁质酶基因的工程植株分析几丁质酶基因的启动子，已鉴定出负责几丁质酶表达的调控序列。

Abstract According to amino acid sequences, plant chitinases fall into six classes (I-VI). Class I chitinases have two domains of conserved sequence separated by a variable hinge region. The first conserved region starting at the N terminal end is a cysteine rich domain of about 40 amino acids, which is involved in chitin binding. The second conserved region is the catalytic structure, located at the C terminal end of the enzymes. Class II chitinases have amino acid sequences similar to the catalytic domain of the class I enzymes, but lack the N terminal cysteine rich domain. Class III chitinases have no sequence similarities with class I and class II chitinases. Class IV chitinases differ from class I in having several deletions in chitin binding and catalytic dormains. Class V chitinases have a duplicated N terminal chitin binding dormain. Class VI chitinases show significant aminoacid sequence similarity to the bacterial chitinases. Chitinase is induced by various factors including pathogens and plant hormons. The regulatory elements of chitinase promotor necessary for ethylene/pathogen regulated expression have been identified. Some chitinases are developmentally regulated in several tissues including flowers, roots and leaves.