In Lower Eukaryotes

For eukaryotic microorganisms, the involvement of PolyPs in biochemical regulation under stress has also been observed. For example, the involvement of vacuolar PolyP in survival under osmotic or alkaline stress has been shown in algae and fungi. In the alga Dunaliella salina, alkalinization of the cytoplasm results in a massive hydrolysis of PolyP, resulting in pH stat. Various authors have suggested that the hydrolysis of PolyP provides the pH-stat mechanism to counterbalance the alkaline stress (Bental et al, 1990 Pick et al, 1990 Pick and Weis, 1991). 

The role of PolyP as a buffer was demonstrated in N. crassa under osmotic stress where the hypoosmic shock produced a rapid hydrolysis of the PolyP with an increase in the concentration of cytoplasmic phosphate (Yang et al, 1993). 

Yeast showed an accumulation of PolyP3 following PolyP hydrolysis induced by amines and basic amino acids (Greenfeld et al., 1987). The degradation of NMR-observable , probably vacuolar, PolyP to short-chain polymers in the cells of Chemostat-cultivated S. cerevisiae contributed to neutralizing the added alkalinity (Castro et al., 1995, 1999). In contrast, when the vacuolar vphl-1 mutant, lacking NMR-visible PolyP, was subjected to alkalinization, the absence of a vacuolar source of phosphate slowed re-acidification (Castro et al., 1999). Anaerobiosis resulted in the complete hydrolysis of PolyP to P (Castro etal, 1995). 

To summarize, in lower eukaryotes the participation of PolyPs in development processes, gene activity control and overcoming stress is confirmed at present by much indirect evidence and thus establishment of the background for such functions is one of the major future tasks in PolyP biochemistry. 

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Heath, LB. (1980). Variant mitoses in lower eukaryotes Indicators of the evolution of mitosis Inti. Rev. Cytol. 64, 1-80. 

Golgi. However, in lower eukaryotes such as fungi and yeast the high-mannose structures are released from the cell as end products of glycosyla-tion (Meynial-Salles and Combes, 1996). 

Catalytic RNAs, or ribozymes, are RNAs, which catalytically cleave covalent bonds in a target RNA. The catalytic site is the result of the conformation adopted by the RNA-RNA complex in the presence of divalent cations. Shortly thereafter, Altman and colleagues discovered the active role of the RNA component of RNase P in the process of tRNA maturation. This was the first characterization of a true RNA enzyme that catalyzes the reaction of a free substrate, i.e., possesses catalytic activity in trans (Guerrier et al. 1983). A variety of ribozymes, catalyzing intramolecular splicing or cleavage reactions, have subsequently been found in lower eukaryotes, viruses, and some bacteria. 

Figure 7.10 The localization and functions of PolyPs in lower eukaryotes.

While expression of biopharmaceuticals in lower eukaryotes such as S. cerevisiae is possible, glycosylation patterns more similar to native human proteins are obtained if expressed in an animal cell Hne. 

Plasmids are circular, double-stranded DNA (dsDNA) molecules that are separate from a cell s chromosomal DNA. These extrachromosomal DNAs, which occur naturally In bacteria and in lower eukaryotic cells (e.g., yeast), exist In a parasitic or symbiotic relationship with their host cell. Like the host-cell chromosomal DNA, plasmid DNA is duplicated before every cell division. During cell division, copies of the plasmid DNA segregate to each daughter cell, assuring con-... 

Uttaro, A.D. Biosynthesis of polyunsaturated fatty acids in lower eukaryotes. lUBMB Life, 58 (2006) 563-571. 

In animal cells, fatty acids are degraded both in mitochondria and peroxisomes, whereas in lower eukaryotes, P-oxidation is confined to peroxisomes. Mitochondrial P-oxidation provides energy for oxidative phosphorylation and generates acetyl-CoA for ketogenesis in liver. The oxidation of fatty acids with odd numbers of carbon atoms also yields propi-onyl-CoA that is metabolized to succinate. 

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