Kharat Arun S, Mahadevan S
Department of Molecular Reproduction, Development and Genetics, Indian Institute of Science, Bangalore 560012, India1.
Microbiology (Reading). 2000 Aug;146 ( Pt 8):2039-2049. doi: 10.1099/00221287-146-8-2039.
The pattern of expression of the genes involved in the utilization of aryl beta-glucosides such as arbutin and salicin is different in the genus Shigella compared to Escherichia coli. The results presented here indicate that the homologue of the cryptic bgl operon of E. coli is conserved in Shigella sonnei and is the primary system involved in beta-glucoside utilization in the organism. The organization of the bgl genes in S. sonnei is similar to that of E. coli; however there are three major differences in terms of their pattern of expression. (i) The bglB gene, encoding phospho-beta-glucosidase B, is insertionally inactivated in S. sonnei. As a result, mutational activation of the silent bgl promoter confers an Arbutin-positive (Arb(+)) phenotype to the cells in a single step; however, acquiring a Salicin-positive (Sal(+)) phenotype requires the reversion or suppression of the bglB mutation in addition. (ii) Unlike in E. coli, a majority of the activating mutations (conferring the Arb(+) phenotype) map within the unlinked hns locus, whereas activation of the E. coli bgl operon under the same conditions is predominantly due to insertions within the bglR locus. (iii) Although the bgl promoter is silent in the wild-type strain of S. sonnei (as in the case of E. coli), transcriptional and functional analyses indicated a higher basal level of transcription of the downstream genes. This was correlated with a 1 bp deletion within the putative Rho-independent terminator present in the leader sequence preceding the homologue of the bglG gene. The possible evolutionary implications of these differences for the maintenance of the genes in the cryptic state are discussed.
与大肠杆菌相比，志贺氏菌属中参与利用诸如熊果苷和水杨苷等芳基β-葡萄糖苷的基因表达模式有所不同。本文给出的结果表明，大肠杆菌中隐秘的bgl操纵子的同源物在宋内志贺氏菌中是保守的，并且是该生物体中参与β-葡萄糖苷利用的主要系统。宋内志贺氏菌中bgl基因的组织与大肠杆菌相似；然而，在它们的表达模式方面存在三个主要差异。（i）编码磷酸-β-葡萄糖苷酶B的bglB基因在宋内志贺氏菌中因插入而失活。因此，沉默的bgl启动子的突变激活可使细胞一步获得熊果苷阳性（Arb(+)）表型；然而，要获得水杨苷阳性（Sal(+)）表型则还需要bglB突变的回复或抑制。（ii）与大肠杆菌不同，大多数激活突变（赋予Arb(+)表型）定位在不连锁的hns基因座内，而在相同条件下大肠杆菌bgl操纵子的激活主要是由于bglR基因座内的插入。（iii）尽管bgl启动子在宋内志贺氏菌的野生型菌株中是沉默的（与大肠杆菌的情况一样），但转录和功能分析表明下游基因的基础转录水平较高。这与bglG基因同源物之前的前导序列中存在的假定的不依赖Rho的终止子内的1个碱基缺失有关。讨论了这些差异对于隐秘状态下基因维持的可能进化意义。
