Transduction chains

Fig. 12. Tentative model of the signal transduction chain that links the perception of pectic fragments to defense responses in carrot cells. Abbreviations apy, heterotrimeric G protein CaM, calmodulin 4CL, 4-coumarate-CoA ligase CTX, cholera toxin FC, fusicoccine GDP-P-S and GTP-y-S, guanosine 5 -0-(2-thiodiphosphate) and guanosine 5 -0-(3-thiotriphosphate) IP3, 1,4,5-inositol trisphosphate PAL, phenylalanine ammonia-lyase PLC, phospholipase C PR, pathogenesis related PTX, pertussis toxin Rc, receptor SP, staurosporine. Activation and inhibition are symbolized by + and -respectively.

The growth rate, however, always returns to the baseline level less than 30 min after the stimulus as shown in Fig. 7. This observation demonstrates that the model is not correct for the growth response as there must be at least two biochemical steps along the light sensory transduction chain which are subject to regulation. One sets the level of sensitivity and the other is responsible for the return of growth rate to the base-line level. Only the first step which sets the level of sensitivity obeys the simple differential equation of Delbriick and Reichardt and only then in cases where a small stimulus was employed. 

For maximizing the sensitivity the transduction chain utilized in the sensor has to be considered. In the MZI this chain is given by Am—>An—>AN, //—>Ay>—>AP ut. If we focus on chemical sensors the measurand m is the concentration of a specific chemical substance and Am can be specified as AC. First step of maximizing the sensitivity now is maximizing the effects of each individual transduction step. 

If the allelochemical is hydrophylic, it cannot enter into the cell and act from outside by binding with chemoreceptors. The compounds from allelopathically active plants may serve as chemosignals and their signalling occurs via alternative pathways (i) Chemoreceptor (sensors) — transducers (G-proteins) —> secondary messengers (Ca2+, cyclic AMP or GMP, inositol triphospate, etc) —> organelles or (ii) Chemoreceptor (sensors) —> ion channels —> action potential organelles, or (iii) Chemoreceptor (sensors) —> ion channels —> cytoskeleton— organelles (Roshchina, 2005 a). What is the effect of acted allelochemical on the pathways, could be analysed to study the effects of substances on separate sites of the transduction chain. 

Fig. 1.36. Regulation of the sub-ceUular localization of the transcription factor SWI5 in yeast by phosphorylation. The subcellular localization of the SWI5 protein is regulated by phosphorylation/ dephosphorylation. In the phos-phorylated state, SWI5 is found in the cytoplasm, while in the under-phosphorylated state it is localized in the nucleus. Phosphorylation and dephosphorylation are catalyzed by either protein kinases or protein phosphatases and can be controlled via signal transduction chains.

The phosphorylation of enzymes by specific protein kinases is a widespread mechanism for the regulation of enzyme activity. It represents a flexible and reversible means of regulation and plays a central role in signal transduction chains in eucaryotes. 

A signal transduction chain can not be viewed as an isolated event within an organism, but should rather be interpreted in the context of other signaling pathways. The cell possesses a large repertoire of mechanisms by which the extent of signal transduction can be regulated and by which different signaling pathways commimicate. Many of these mechanisms will be dealt with in detail in later chapters. 

The receptors for cytokines and interferons are the starting point for signal transduction chains that bring about an activation of transcription factors. The signaling pathway involves the Janus protein kinase and Stat transcription factors (see 11.1.4). Phosphoty-rosine-SH2 interactions are also involved in several steps of signal transduction here. 

Rg.3.1 Common structural motifs, instrumental in forming interconnectivity of components in signal transduction chains. 

It has been known for a long time that a number of enzymes are regulated by insulin through phosphorylation and dephosphorylation at serine residues (Kahn, 1985). Therefore, a signal transduction from the tyrosine-specific insulin receptor kinase to a serine-specific kinase must occur. The serine kinase that might fulfil both functions in the insulin signal-transduction chain has not yet been identified however, there are several possible candidates for these so called switch kinases (Fig. 10). 

More than 100 dominant oncogenes have been identified to date. Nearly all the components of the signal transduction chains that transmit signals from the cell exterior to the level of the cell cycle and transcription can be converted by mutations into an oncogenic state. It has to be emphasized that activation of an oncogene is generally not sufficient for transformation of a normal cell into a tumor cell. As outlined in Section 14.8.6, functional inactivation of cell death mechanisms must occur at the same time, and it is the cooperation of defects in oncogenic pathways and in apoptotic pathways that paves the way for the formation of a real tumor. 

Quite apart from the complexity introduced by the possible involvement of SMG proteins, protein kinase cascades themselves are extremely sophisticated and it seems likely that this sophistication will extend to plants. This is not the place to discuss the ramifications of these systems since the biochemical and genetic evidence is not yet available in plants and the reader is directed to several comprehensive reviews [57,58,71,81] but an illustration of the possible complexities is outlined in Fig. 5. The points at which there is some evidence of involvement in the ethylene transduction chain are indicated. 

For C. roseus suspension-cultured cells, elicitation with fungal elicitors results in the induction of TDC activity (99,186,202,203,284,329-331). This is due to the induction of expression of the Tdc gene. Similarly, SSS activity is induced (202,203,284,329,330). The induction by the Pythium aphanider-matum or yeast elicitor of the transcription of both genes is not affected by cycloheximide that is, the induction is independent of de novo protein biosynthesis, and thus follows an already available signal-transduction chain. The response is quite fast, for the enhanced transcription can already be measured 15 min after elicitation (202,203). Also, the NADPH cytochrome P-450 reductase mRNA level is induced by elicitation with fungal elicitors (113). Moreno et al (99,151) measured activities of a number of enzymes involved in secondary metabolism in C. roseus before and after elicitation with a P. aphanidermatum preparation. GlOH activity was found to be slightly decreased by elicitation and IPP-isomerase showed similar behavior. The pattern of terpenoids formed by the crude enzyme extracts from elicited and nonelicited cells was different. The total incorporation decreased, that is, the activities of the enzymes of the terpenoid pathway were lower. The relative incorporation decreased particularly for squalene. 

Jasmonate is a well-established endogenous signal compound in the signal-transduction chain in plant development and in the response to... 

Phototaxis in Rb. sphaeroides is triggered by the effects of the photosynthetic machinery on the rate of electron transfer (9). Thus, in contrast to the situation in H. salinarum, phototaxis in Rb. sphaeroides does not involve a dedicated photosensor. The rate of electron transfer is presumably sensed by an as yet unidentified receptor, and relayed into the complex set of Che proteins in Rb. sphaeroides (Fig. 1). Thus, phototaxis responses in Rb. sphaeroides can be regarded as a form of redox taxis and are modulated by factors affecting electron transport, such as the presence or absence of oxygen (19). The Rb. sphaeroides encodes nine transmembrane chemoreceptors (MCPs) and four putative cytoplasmic MCPs, four CheA proteins, and six CheY proteins (20). A number of proteins from this Che system have been shown to be required for phototaxis in Rb. sphaeroides, showing that the signal transduction chains for phototaxis and chemotaxis converge at this level (21). A similar situation holds for phototaxis and chemotaxis in R. centenum (see later). 

Motile microorganisms respond to a multitude of stimuli in their environment to select favorable habitats suitable for growth and survival as well as reproduction. These intracellular sensory transduction chains triggered by the receptors are linked intracellu-larly, resulting in a vectorial addition of the response paths. In other cases, the reaction to one stimulus may override that to others e.g., the reaction to strong light may override gravitaxis to avoid detrimental radiation at the water surface. 

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