摘要
大肠杆菌棉子糖操纵子(raf)位于质粒,其第一结构基因rafA编码的α-半乳糖苷酶为诱导酶。raf操纵子比乳糖操纵子(lac)或蜜二糖操纵子(mel)对诱导物有更严格的结构特异性。该酶被蜜二糖或棉子糖诱导,也被D-半乳糖微弱诱导,但不受乳糖、PNPG等结构相近糖所诱导。α-半乳糖苷酶的酶诱导形成能力在对数生长末期出现高峰。raf操纵子基因结构组成及调节与乳糖操纵子相似。以阻遏物为中介的负调控在raf操纵子调节中起主要作用,同时以环腺苷-代谢降解物基因激活蛋白(cAMP-CAP)为中介的正调控也参与调节。当0.4%葡萄糖加入到其它碳源培养基时,该酶表达水平下降至原活力的1/2—1/3。无论诱导或组成型酶的葡萄糖抑制均未见瞬时抑制。腺苷环化酶(cya)缺失或环腺苷受体蛋白(crp)和cya双缺陷菌株的酶表达则分别下降到原活力的9%和2.5%。cya突变株或葡萄糖对raf操纵子表达的抑制可被cAMP解除,但cya和crp双缺陷菌株仍有葡萄糖抑制,而且这种抑制不为cAMP抵消,表明通过降低cAMP而影响cAMP-CAP复合体形成还不能解释代谢降解物抑制的全部机制。尚无证据说明吲哚类小分子化合物和低浓度尿素对raf操纵子表达的明显作用。
The α-galactosidase, coded for by the first structural gene rafA in the plasmid determined raf operon was an inducible enzyme. In contrast to lac or mel operon, raf operon has more strict structural specificity for inducers. The enzyme can be induced by melibiose and raffinose, or weakly by D-galactose, but not by structurally related sugars such as lactose, PNPG etc.. The α-galacosidase forming capacity as function of growth curve reached a single peak at the end of the logarithmic phase of the growth. The structure and regulation of raf operon is similar to those of lac operon. The repressormormediated negative control plays a major role in the regulation of raf operon, and cAMP-CAP mediated positive control is also involved in the regulation.When 0.4% glucose was added into the medium with other carbon sources, the expression of the enzyme was repressed by 2—3 fold. Transient catabolite repression has been observed neither in inducible nor constitutive α-galactosidase expression. Based on α-galacosidase assay, in mutant strains CA8306(cya) and CA8445 (cya, crp) the expression level of raf operon was only 9% and 2.5% of that in wild type strain respectively. The glucose effect or the repression in cya mutant can be abolished by 1—5 mmol cAMP. The constitutive α-galacotsidase expression in cya and cry double mutant (CA8445) remains repressible by glucose, but irreversible by cAMP, suggesting cAMP-CAP complex is not the exclusive mediator of the catablite repression. No evidence showed any significant effect on raf operon by either indole derivatives or low concentration of urea as previous works reported in some other operons related to carbon source metabolism.
出处
《微生物学报》
CAS
CSCD
北大核心
1989年第3期180-186,共7页
Acta Microbiologica Sinica
