Repression by glucose of acetohydroxy acid synthetase in Escherichia coli B

Acetolactate formation in Escherichia coli B results from the activity of a single system, acetohydroxy acid synthetase, which has a pH optimum of 8.0 and is sensitive to end-product inhibition by l-valine. Acetohydroxy acid synthetase was found to be subject to catabolite repression, and the nature and concentration of the carbon source had a greater effect on the formation of the enzyme than had the known end products (valine, isoleucine, leucine and pantothenate) of the biosynthetic pathways of which this enzyme is a member. The results suggest that acetohydroxy acid synthetase may play an amphibolic role in E. coli B.     

Repression of galactose utilization by glucose in the citrate-producing yeastCandida guilliermondii

Summary A strain of the yeastCandida guilliermondii has been shown to produce citric acid from galactose to a similar extent, and at a similar rate, as from glucose. At an initial concentration of 36 g/l of either glucose or galactose, citric acid production exceeds 13 g/l. When galactose and glucose are present in a mixture, however, galactose utilization is delayed until most of the glucose has been utilized, providing evidence for catabolite repression.     

Repression and inactivation by glucose of the maltose transport system of Candida utilis

Summary The maltose utilization system of Candida utilis was affected by glucose through two different mechanisms: catabolite repression and inactivation. Maltose permease was under the control of both, whereas -glucosidase was only repressed.In glucose-maltose continuous culture, both sugars were consumed simultaneously at glucose steady-state concentrations in the fermentor below 100 mg/l, corresponding to dilution rates lower than 0.4 h^-1. At higher dilution rates, and consequently higher glucose concentrations, repression increased steeply, being complete when glucose concentration reached 170 mg/l.Glucose induced inactivation of maltose permease, in maltose-growing and resting cells, by decreasing V _max, without changing maltose affinity for its transport system. The inactivation process apparently required the entrance of the inactivator into the cell and its subsequent phosphorylation because: 1) The specific inactivation rate showed a dependence on glucose similar to that of glucose transport and 2) only rapidly phosphorylated glucose analogues could mimic the inactivation effect.     

Regulation of Aconitase Synthesis in Bacillus subtilis: Induction, Feedback Repression, and Catabolite Repression

The synthesis of aconitase in Bacillus subtilis wild-type and different citric acid cycle mutants has been studied and the influence of various growth conditions examined. Aconitase is induced by citrate and precursors of citrate and repressed by glutamate. Induction and repression counteract each other, and at equimolar concentrations of citrate and glutamate, aconitase synthesis is unaffected. Induction by citrate can partly overcome catabolite repression of aconitase. Isocitrate dehydrogenase show endogenous induction of aconitase due to citrate accumulation. Leaky mutants defective in citrate synthase and aconitase cannot be induced by citrate, which indicates that they carry a regulatory mutation. The complex regulation of aconitase is discussed with reference to the participation of this enzyme in glutamate biosynthesis and energy metabolism.     

Repression of Staphylococcus aureus in Associative Culture

The growth of Staphylococcus aureus MF 31 was suppressed when grown in association with Streptococcus diacetilactis and other lactic streptococci. The data indicated that the initial proportion of staphylococci present in the medium was of less importance than the depletion of vital nutrients. Investigation revealed that factors present in Yeast Nitrogen Base medium could reverse the inhibition which was due to antagonism. The major factor found was nicotinamide, and further study revealed that the biological availability of this compound was influenced by the pH of the medium. The addition of nicotinamide to depleted media with careful control of the pH resulted in increased growth of S. aureus in the test system. Further data indicated that additional factors may affect the degree of growth by S. aureus in depleted media.     

Repression of MHC determinants in HIV infection

HIV infects CD4(+) macrophages and lymphocytes. Before the development of AIDS, HIV weakens the immune system in part by blocking antigen processing and presentation via major histocompatibility complex (MHC) molecules. In this report, we discuss how HIV escapes the immune surveillance by MHC II molecules.     

SUMO在转录中的抑制作用

许多调控基因转录的重要蛋白质能被SUMO (small ubiquitin-related modifier)化修饰,这些蛋白质包括转录因子,转录辅助因子和染色质修饰酶.SUMO化修饰对底物蛋白的活性产生影响,在大多数情况下,与转录活性的抑制有关.最近,对SUMO化调控转录的机制有了新的认识,认为SUMO化的一个重要作用是促进转录因子与转录抑制因子之间的相互作用.另一方面,已经发现转录共抑制因子HDAC (组蛋白去乙酰化酶)可以作为SUMO化的底物、效应因子和调控因子,说明乙酰化和SUMO化之间复杂的相互作用对基因转录调控起着非常重要的作用.     

种子萌发的抑制调控机制

种子萌发是植物生命周期中一个重要的生理过程,激素作用、miRNA抑制、mRNA区域化、表观遗传调控等多个层次的分子抑制参与该过程的调控。赤霉素（解除抑制的激素）合成和失活的调控主要发生在转录水平,而脱落酸（引起抑制的激素）信号转导途径的调控则通过蛋白质抑制物的降解来实现。miRNA在转录后水平使其靶基因的mRNA降解,抑制种子的萌发;通过mRNA的区域化抑制与萌发相关基因的翻译属于另一层次的转录后抑制;小RNA介导的表观遗传机制也可能在种子萌发过程基因表达的协同调控中发挥重要作用。与分子水平的抑制类似,胚乳和种皮产生的机械抑制也很重要。