Translation adenylation

A poly(A) "tail" consisting of -250 residues of adenylic acid is added next by poly(A) polymerase, a component of an enzyme complex that also cleaves the RNA chains.545 57111 Most eukaryotic mRNA is polyadenylated with the exception of that encoding histones. The function of the poly(A) is unclear. It is needed for transport of mRNA out of the nucleus, but it does confer a greatly increased stability to the mRNA in the cytoplasm where the adenylate irnits are gradually removed.307 308 In contrast, in chloroplasts and plant mitochondria polyadenylation is required for rapid degradation of mRNA.571c d Polyadenylation may also increase the efficiency of translation.572 Polyadenylation occurs rapidly within -1 min after transcription is completed. 

In other cell types, guanine nucleotides interact with a guanine nucleotide subunit (G- or Ng-subunit) to translate receptor stimulation into increased adenylate cyclase activity (12.) Cholera toxin inhibits a specific GTPase on this guanine nucleotide subunit and thereby increases adenylate cyclase activity (13.). In dispersed cells from the bovine parathyroid gland, cholera toxin markedly increases cAMP formation and causes a 3 to 10-fold increase in the apparent affinity cf dopamine for its receptor (as determined by cAMP accumulation or IR-PTH secretion (J y.). The effects of guanine nucleotides and cholera toxin on cAMP accumulation in parathyroid cells result from interactions with the guanine nucleotide subunit in this cell. 

Among activated forms of amino acids, mixed anhydrides with inorganic phosphate or phosphate esters require a special discussion because they are universally involved in peptide biosynthesis through the ribosomal and non-ribosomal pathways. These mixed anhydrides have stimulated studies in prebiotic chemistry very early in the history of this field. Amino acyl adenylates 18c have been shown to polymerize in solution [159,160] and in the presence of clays [139]. However, their participation as major activated amino acid species to the prebiotic formation of peptides from amino acids is unlikely for at least two reasons. Firstly, amino acid adenylates that have a significant lifetime in aqueous solution become very unstable as soon as either CO2 or bicarbonate is present at millimolar concentration [137]. Lacey and coworkers [161] were therefore conduced to consider that CO2 was absent in the primitive atmosphere for aminoacyl adenylate to have a sufficient lifetime and then to allow for the emergence of the modern process of amino acid activation and of the translation apparatus. But this proposition is unlikely, as shown by the analysis of geological records in favor of CO2 contents in the atmosphere higher than present levels [128]. It is also in contradiction with most studies of the evolution of the atmosphere of telluric planets [30,32], Secondly, there is no prebiotic pathway available for adenylate formation and ATP proved to be inefficient in this reaction [162]. 

This K is considerably smaller than that of the single-strand polymer at neutral pH, and suggests that the molecules have a rather thick structure. Adopting the interrupted-helix model, we may apply Eq. (97) as in the case of poly-L-proline. Noting that the unit translation distance b0 is 1.70 A for the double-stranded Watson-Crick helix (263 ) [i. e., a distance of 3.40 A for two residues, one in each chain], and taking Mu = 328 for poly(adenylic acid), we obtain r = 22 residues per helical... 

It has been shown that the first step in ribosome-dependent peptide synthesis is activation of amino acids to form amino acid adenylates. The amino acids are then transferred to RNA present in the soluble extract of the cell, the so-called transfer RNA (tRNA) to which the amino acids become fixed by an ester linkage. These two steps are usually referred to as the formation of aminoacyl-tRNA. The next step, the translation step of codons in messenger RNA (mRNA), which is associated with ribosomes, to provide a polypeptide includes three stages (1) chain initiation by mutual coordination with initiation factors, (2) chain elongation in aid of elongation factors, and (3) chain termination in support of release factors. 

Phosphoenolpyruvate carboxykinase is induced. Oxaloacetate produces PEP in a reaction catalyzed by PEPCK. Cytosolic PEPCK is an inducible enzyme, which means that the quantity of the enzyme in the cell increases because of increased transcription of its gene and increased translation of its mRNA. The major inducer is cyclic adenosine monophosphate (cAMP), which is increased by hormones that activate adenylate cyclase. Adenylate cyclase produces cAMP from ATP. Glucagon is the hormone that causes cAMP to rise during fasting, whereas epinephrine acts during exercise or stress. cAMP activates protein kinase A, which phosphorylates a set of specific transcription factors (CREB) that stimulate transcription of the PEPCK gene (see Chapter 16 and Pig. 16.18). Increased synthesis of mRNA for PEPCK results in increased synthesis of the enzyme. Cortisol, the major human glucocorticoid, also induces PEPCK. 

An artificial ribozyme mimics this translation step of the ribosome. The spedlity of this selected ribozyme is based on the recognition of an adenosine moiety of the amino acid ester and allows the utilization of leucine- and phenylalanine- as weU as methionine-derivatized substrates. This tolerance for various amino acids indicates the possibility of selecting more general ribozymes for protein synthesis. Furthermore, a related ribozyme efficiently catalyses the synthesis of -30 different dipeptides from an aminoacyl-adenylate substrate. Ribozyme-mediated synthesis of uncoded peptides might have been an important step in the transition from a RNA to a peptide world before the anergence of the ribosome. ... 

Experiments were then performed to test the validity of the concept of spare receptors as the explanation of the alteration in value during desensitization in NCB-20 cells. It was shown first that desensitization to PGI2 responsiveness was mediated by a relatively rapid loss of membrane receptors k = 5.08 x 10 s ). In other experiments, however, cells were cultured in the presence of cycloheximide (an inhibitor of ribosomal translation that arrests totally new protein synthesis) which resulted in a substantially slower loss of both adenylate cyclase molecules as measured by basal enzyme activity k = 1.94 X 10" s ), and PGI2 receptors (fc = 0.92 x 10 s ). 

In the present study, biosynthesis of both enzyme forms has been studied using heterotrophic tissues of Pisum sativum L. For this purpose, cotyledons of germinating or developing seeds were chosen. Poly-adenylated RNA was isolated and translated in vitro. Phosphorylase isozymes were identified by immunoprecipitation and SDS-PAGE. Following in-vitro translation, the size of the cytosolic isozyme was indistinguishable from that of the mature protein. In contrast, the plastidic isozyme was synthesized as a precursor which was processed to its final size by a stromal fraction of isolated pea chloroplasts. 

The adenylate cyclase system is a complex of proteins which responds to several stimuli with a single, integrated response. A hormonal signal is translated by multiple systems, including the stimulation of cAMP, to the nucleus [1,2]. Regulation of cAMP synthesis in response to a hormone requires GTP as a cofactor for hormonal stimulation of the adenylate cyclase system [3,4]. GTP appears critical for the action of various toxins, such as cholera and pertussis toxin, as well as for hormonal activity [5,6]. 

Yuan, W. Yang, J. Kopefikovd, P. Kopecek, J. Smart hydrogels containing adenylate kinase translation substrate recognition into macroscopic motion. /. Am. Chem. Soc. 2008,10, 15760-15761. 

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