Signal Transduction and Gene Regulation - Lecture Notes | HSCI 4607, Study notes of Health sciences

Download Signal Transduction and Gene Regulation - Lecture Notes | HSCI 4607 and more Study notes Health sciences in PDF only on Docsity! Bacterial Phys HSCI 4607/5607 Ch: 18 Signal Transduction and Gene Regulation Introduction: Bacteria have ability to adapt and survive ever-changing environmental conditions. This is due to the presence of sophisticated signaling systems that transmit signal across the cell membrane to specific intracellular targets, which can be transcriptional machinery, enzymes, or a cellular components. In most cases the signal causes the cells to activate or inactivate a cytoplasmic transcription factor. This activation or inactivation of transcription factor occurs usually via either covalent modification of the transcription factor, for example, phosphorylation or dephosphorylation or conformational changes upon binding to a co-inducer or repressor. There are several different mechanism of signal transduction that includes: 1. Two component signaling system; 2. The FNR system; 3. The formate regulon system; 4. catabolite repression system in both gram-negative and gram-positive bacteria; and 5. Acylated homoserine lactone system. cytoplasm cell membrane to target  Examples of two-component System: The effect of phosphorylated response regulator is usually stimulation or repression of the gene transcription but some response regulator do both depending upon the target gene; for example NarL, a response regulator involved in nitrate/nitrite reduction reactions. NarX which is a membrane bound kinase, phosphorylates NarL to NarL-P in the presence of nitrate, which in turn stimulates NarG gene to express nitrate reductase. While in the presence of excess of nitrite NarX dephosphorylates NarL and thus reduces the level of NarL-P repressing the expression of NarG gene. Some histidine kinases also function by dephosphorylation, for example, PhoR/PhoB system where excess of inorganic phosphate stimulates the histidine kinase PhoR to dephosphorylate the response regulator PhoB repressing the transcription of the phosphate utilization genes known as PHO regulon. Acetyl Phosphate as a Possible Global Signal in some two- component systems: It has been proposed that in the absence of cognate histidine kinase (HK) proteins, acetyl phosphate appears to be able to donate the phosphoryl group to the cognate response regulator protein replacing the HK activity. It is not known whether acetyl phosphate does the same even in the cells that do not lack HK proteins. The Formate Regulon System: In E.coli when it is grown anaerobically, formic acid is converted in to H2 and CO2 by enzyme complex formate hydrogen- lyase. The complex consists of two enzymes, formate dehydrogenase H and hydrogenase 3. These enzymes are repressed by oxygen and nitrate while induced by formate. The reaction is catalyzed at acidic pH, thus it prevents the increasing acidity by degrading formic acid. The formate hydrogen-lyase genes are part of format regulon which is also known as FhlA regulon. The regulon is controlled by a transcription factor called FhlA and by formate which is a coactivator of FhlA. Under aerobic condition since the level of formate is kept low the regulon remains repressed. Formate Regulon….. There are two ways in which formate level is kept low under aerobic conditions: 1. The enzyme pyruvate formate-lyase (pfl gene) which synthesizes formate and Acetyl-CoA from pyruvate and CoASH requires FNR protein as positive regulator, the activation of which is prevented by oxygen. 2. Oxygen and nitrate also lower the level of formate in the cells by inducing synthesis of respiratory formate dehydrogenases, which oxidizes formate to CO2. Response to carbon source:Catabolite Repression: Catabolite repression refers to the preferential use of one type of carbon source over another when organism is grown in the presence of two carbon sources. As described in growth chapter, it leads to diauxic growth. There are more than one type of mechanism involved here. One of the mechanism is that the glucose transport by phosphotransferase system (PTS) in E.coli lowers the cAMP pool while cAMP acts as a positive regulator for many alternative carbon sources. Glucose uptake by PTS system also inhibit permease required for the uptake of other carbohydrate carbon sources ( inducer exclusion). There are two more systems which are responsible for catabolite repression: 1. Cra system which does not require either cAMP or PTS. 2. CRE system of Bacillus subtilis. Inducer Expulsion: Glucose prevents the accumulation of other sugars in some gram- positive bacteria. This is not via inhibition of uptake as in E.coli but by two different mechanisms which lead to the equilibration of sugar across the membrane and thus not allowing accumulation. One way is to uncouple sugar transport from the proton motive force and the second is the dephosphorylation of the incoming sugar-P. For example, in Lactobacillus brevis, Hpr(Ser-P) appears to bind to the lactose/proton symporter and uncouples sugar transport from proton transport. The consequence of uncoupling is that now the symporter catalyzes facilitated diffusion rather than active transport and thus no accumulation of lactose. In some gram-positive bacteria like streptococci, lactococci, and enterococci, there is a presence of membrane-associated phosphatase activated by Hpr(ser-P). It is suggested that when sugar phposphates are accumulated in the cell via PTS system, they are dephosphorylated and exit along their concentration gradient. This process is termed as ‘inducer expulsion’. Quorum-Sensing Systems: Quorum sensing refers to the production of extracellular molecules that signal the population cell density. In gram-negative bacteria acylated homoserine lactones are the quorum sensing signals produced by high-density populations. But some bacteria also use lipid or amino acids as signals. In gram-positive B.subtilis peptides signal cell density. Accumulation of peptides in the extracellular medium signal cell density. Quorum sensing using Acylated Homoserine Lactone(HSL): When the population of bacteria reach critical cell density threshold, the accumulated acylated HSLs generally induce the expression of certain genes that are only active at high cell densities. The first quorum sensing systems based upon acylated HSLs were discovered in luminescent bacteria where they serve to activate the genes for luminescence. Now it is known that such systems are wide spread in different bacteria. Acylated HSL system in luminescent bacterium Vibrio fischeri: It is known that certain species of luminescent bacteria will emit light only when cell density reaches at certain threshold (quorum), such as > 107 cells/ml. This is because the genes required for luminescence are not activated until the concentration of specific signal known as ‘autoinducer’ reaches certain level. Acylated HSLs act as autoinducer. The genes that code for leuciferase proteins and fatty acid reductase are part of ‘lux’ operon which consists of five genes luxCDABE. The expression of these genes are regulated by the autoinducers.