Eukaryotic systems

Methyl parathion has been tested in numerous genotoxicity assays using prokaryotic and eukaryotic systems with both positive and negative results. Results of these studies are summarized in Tables 3-5 and 3-6. 

We shall now turn to animals to show how they, with fungi, that is the total living non-photosynthetic multi-cellular eukaryote system, have developed in step with and dependent upon the evolution of plants much as we stressed the evolutionary... 

Volume 306. Expression of Recombinant Genes in Eukaryotic Systems Edited by Joseph C. Glorioso and Martin C. Schmidt... 

Most of the recombinant subunit vaccines tested in the first half of this decade employed gp 120 or gp 160 expressed in yeast, insect or mammalian (mainly CHO) cell lines. Eukaryotic systems facilitate glycosylation of the protein products. Like all subunit vaccines, these stimulate a humoral-based immune response but fail to elicit a strong T-cell response. The failure to elicit a cell-based... 

The three major classes of biopolymers found in eukaryotic systems are nucleic acids, proteins, and polysaccharides. The latter class is the most complex with respect to structural and stereochemical diversity. Polysaccharides indeed possess a massive information content. Furthermore, polysaccharides are commonly found in nature covalently attached (conjugated) to other biomolecules such as proteins, isoprenoids, fatty acids, and lipids.1... 

Other proteins also participate in the integration process. One class is composed of molecular chaperones such as SecA in bacteria and Hsp70 or BiP in eukaryotes (Qi and Bernstein, 1999 Schekman, 1994 Mothes et al., 1997 Hamman et al., 1998 Pilon and Schekman, 1999). Another important player in the eukaryotic system is TRAM (translocating chain-associating membrane protein) (Walter, 1992). 

Most of the arguments described in the sections on bacterial signal peptides and membrane proteins seem to be valid for the eukaryotic systems, as well as the translocation phenomena across the ER membrane (Sakaguchi, 1997). They seem to be also true for the translocation system across the mitochondrial inner membrane protein into the intermembrane space and the system across the thylakoid membrane in chloroplasts. Although the TAT-dependent pathway has not been found in the ER, it exists on the thylakoid membrane (and possibly on the inner membrane of mitochondria). 

As described in Section II,C,2, some differences exist between the bacterial and eukaryotic systems on the multispanning membrane assembly (Gafvelin et al., 1997) however, they also have many points in common the multispanning membrane proteins are likely to be co-translationally integrated (Ulbrandt et al., 1997), and both systems use homologous translocon channels, which play an important role for the topogenesis of these multispanning membrane proteins (Prinz et al., 1998). 

Post-translational modifications, such as phosphorylation, complex glycosylation, and lipidation, typically occur in eukaryotic organisms. Therefore, their expression in prokaryotic systems like Escherichia coli is difficult. However, it should be noted that via clever engineering and coexpression of specific enzymes, access can be granted to specific lipidated proteins via expression in bacteria, for example, via the expression of A -myristoyltransferase in E. coli Eukaryotic systems that can be used for the expression of post-translationally modified proteins are yeast and Dictyostelium discoidum. Furthermore, lipidated proteins, such as the Rah proteins, can be obtained via purification from tissue sources or from membrane fractions of insect cells that had been infected with baculovirus bearing a Rah gene. ... 

Studies on the genotoxicity of methoxychlor have generally yielded negative results in prokaryotic assays, mixed results in in vitro eukaryotic systems, and negative results in in vivo studies. ... 

Mustard gas is highly genotoxic. In vitro assays in both prokaryotic and eukaryotic systems support a mechanism of DNA alkyla-... 

Eukaryotic systems have four separate systems that repair double-stranded DNA breaks induced by radiation (62) ... 

Na /Mg +/Cl ions were used instead. Temperatures in the range 20-28°C and a pH value of 7.5 were favored. Jensen and Fenical [71] recommended replacing the above traditional nutrient components by natural C and N sources, polysaccharides and proteins from marine eukaryotic systems. 

Vogel, E. and Natarajan, A.T. The relation between reaction kinetics and mutagenic action of monofunctional alkylating agents in higher eukaryotic systems Interspecies comparisons. IN deSerres, F.J. and Hollaender, A., eds. Chemical Mutagens Principles and Methods for Their Detection, Volume 7. New York Plenum Press. 1982. p. 295-336. 

Similar conventions exist for the naming of eukaryotic genes, although the exact form of the abbreviations may vary with the species and no single convention applies to all eukaryotic systems. 

Protein-DNA binding interactions are the basis of the intricate regulatory circuits fundamental to gene function. We now turn to a closer examination of these gene regulatory schemes, first in prokaryotic, then in eukaryotic systems. 

As we have indicated, the codon AUG is the only one generally used to specify methionine, but it serves a dual function in that it is also used to initiate translation. Occasionally, GUG and UUG are also read as an initiating codon in bacteria, but in internal positions these codons are always read as valine and leucine, respectively. In eukaryotes, initiation at codons other than AUG is much less frequent than in prokaryotes. Weak initiation occasionally occurs at GUG, CUG, and ACG codons in eukaryotic systems. The UGA triplet also serves a dual function it is usually recognized as a stop, but on occasion it serves as a codon for selenocys-teine (box 29A). 

In prokaryotic systems, a specific formylating enzyme exists that can recognize tRNAfMet and formylate, its amino terminus utilizing N1 H-formyItetrahydrofolatc as the formyl donor. This reaction serves to ensure that the initiator does not participate in the elongation reactions. For unknown reasons this recognition step is not possessed by eukaryotic systems. 

At the conclusion of the initiation process, the ribosome is poised to translate the reading frame associated with the initiator codon. The translation of the contiguous codons in mRNA is accomplished by the sequential repetition of three reactions with each amino acid. These three reactions of elongation are similar in both prokaryotic and eukaryotic systems two of them require nonribosomal proteins known as elongation factors (EF). Interestingly, the actual formation of the peptide bond does not require a factor and is the only reaction of protein synthesis catalyzed by the ribosome itself. 

The final step in elongation is known as translocation (fig. 29.17). This reaction, like aminoacyl-tRNA binding, is catalyzed by a factor (the translocation factor, known as EF-G in prokaryotic systems and EF-2 in eukaryotic systems) that cycles on and off the ribosome and hydrolyzes GTP in the process. The overall purpose of translocation is to move the ribosome physically along the mRNA to expose the next codon for translation. 

The structural differences between bacterial and eukaryotic elongation factors are highlighted by the selective action that diphtheria has on eukaryotic systems (box 29B). 

Hershey, J. W. B., Protein phosphorylation controls translation rates. J. Biol. Chem. 264 20823, 1989. Describes how protein kinases are thought to regulate translation in eukaryotic systems. 

Compare the translation initiation signals in prokaryotic and eukaryotic systems, and describe those features of each type of mRNA that determine the frequency with which a particular message is translated. What consequences do these differences have for gene organization in the two systems ... 

Still, trans-acting regulatory proteins that bind to cis effector sites on the genome are present in eukaryotic systems as well as in E. coli. The best understood unicellular eukaryote is the budding yeast Saccharomyces cerevisiae. Gene regulation, particularly of development, can be quite complex in multicellular eukaryotes. Our discussion in this chapter focused on the following points. 

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