Molecular and Cell Biological studies on Intracellular Symbiosis

1.共生蛋白是一种能量偶联蛋白，在高能磷酸化合物存在下发生自磷酸化，并将其磷酸基团转移到其他受体化合物上。虽然共生素的这一特性使人想起在大肠杆菌中传递环境信号的双组分系统中的组氨酸蛋白激酶，但这两种蛋白质之间并没有明显的同源性。有人认为，共生素在HIS-133上被磷酸化，而GroEL有ALA而不是他的。由于这两个蛋白质之间的氨基酸序列高度保守，因此更明显的是，由于GroEL中Ala密码子的连续三次碱基替换，共生素在这个位置创造了His。共生素可能通过正选择获得了新的功能，有利于该位置的His，这表明适应性进化甚至在分子水平上也起作用。而在Groe操纵子中，Groes和GroEL的表达水平相当，而在蚜虫内共生体中，Groes同源基因的表达不成比例地低于共生素的表达。在内生菌利用谷氨酰胺合成必需氨基酸的过程中，谷氨酰胺首先在细菌细胞内转化为谷氨酸和氨，然后被共生菌吸收。蚜虫的一种肠道微生物类似于大肠杆菌，但比后者更喜欢较低的温度。几个基因的结构比较表明，肠道微生物比大肠杆菌在系统发育上更接近内共生体。

1. Symbionin, an aphid endosymbiont-specific protein, is an energy-coupling protein which undergoes autophosphorylation in the presence of high-energy phosphate compounds and transfers its phosphate group to other acceptor compounds. While this property of symbionin is reminiscent of that of histidine protein kinases in the tow-component system that transduces environmental signals in E.coli, no considerable homology between the two proteins is apparent.2. It has been suggested that symbionin is phophorylated at His-133, where GroEL has Ala instead of His. Since between the two proteins the overall amino acid sequence is highly conserved, it is all the more conspicuous that symbionin has created His at this position as the result of three consecutive base substitutions of a codon for Ala in GroEL.It is possible that symbionin has acquired a new function through positive selection in favor of His at this position, suggesting that adaptive evolution is in action even at molecular level.3. While in the groE operon the expression of GroES and GroEL are comparable with each other, in the aphid endosymbiont the expression of GroES homolog is disproportionately less than that of symbionin.4. In the synthesis of essential amino acids by the endosymbiont using glutamine, glutamine is first converted into glutamate and ammonia in the bacteriocyte and then absorbed by the symbiont.5. One of gut microbes of aphid resembles E.coli, but prefers lower temperatures than does the latter. Comparison of structure of several genes suggested that the gut microbe is phylogenetically closer to the endosymbiont than is E.coli.