Yeast nucleus

The cellular distribution of 26S proteasome subunits in yeast appears to be similar as in higher eukaryotes but there also exist some differences in the details. Indirect immunoflourescence studies localized the GFP-tagged yeast 26 proteasome primarily to the yeast nucleus and the nuclear periph-... 

Research work with large genomes and the associated need for high-capacity cloning vectors led to the development of yeast artificial chromosomes (YACS Fig. 9-8). YAC vectors contain all the elements needed to maintain a eukaryotic chromosome in the yeast nucleus a yeast origin of replication, two selectable markers, and specialized sequences (derived from the telomeres and centromere, regions of the chromosome discussed in Chapter 24) needed for stability and... 

Fames, P. and J Beggs The Yeast Nucleus, Oxford University Press, Inc, New York, NY, 2000. 

For these reasons, it has been difficult to develop procedures that allow cosmid probes to be detected reliably in whole yeast cells. Other investigators have demonstrated that such probes may be used if the cells are subjected to heavy extraction with detergents, proteases, and other agents, effectively spreading the yeast nucleus onto microscope slides. However, to attain the goal of whole-cell preservation has required a different approach. 

Sharma PK, Baruah P (1975) Effect of auxins on yeast. Nucleus 18 124-128 Shen-Miller J, McNitt RE, Wojciechowski M (1978) Regions of differential cell elongation and mitosis, and root meristem morphology in different tissues of geotropically stimulated maize root apices. Plant Physiol 61 7-12... 

Fig. 1.10. The yeast nucleus (Williamson, 1991). SPB = Spindle pole body NUC = Nucleolus = Pore CHR = Chromatin CT = Continuous tubules DCT = Discontinuous tubules CTM = Cytoplasmic microtubules...

Only one plasmid, called the 2 xm plasmid, has been identified in the yeast nucleus. It is a circular molecule of DNA, containing 6 kb and there are 50-100 copies per cell. Its biological function is not known, but it is a very useful tool, used by molecular biologists to construct artificial plasmids and genetically transform yeast strains. 

The smaU nucleus of the yeast ceU is surrounded by a membrane or tonoplast, which has many pores with an average diameter of about 0.085 p.m. 

Yeasts are one land of fungi. They are unicellular organisms surrounded by a cell wall and possessing a distinct nucleus. With veiy few exceptions, yeasts reproduce by a process known as budding, where a small new cell is pincEed off the parent cell. Under certain conditions, an individual yeast cell may become a fruiting body, producing spores. 

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

The Rieske protein in mitochondrial bci complexes is assembled when the protein is incorporated into the complex. The Rieske protein is encoded in the nucleus and synthesized in the cytosol with a mitochondrial targeting presequence, which is required to direct the apoprotein to the mitochondrial matrix. The C-terminus is then targeted back to the outside of the inner mitochondrial membrane where the Rieske cluster is assembled. In addition, the presequence is removed and the protein is processed to its mature size after the protein is inserted into the bci complex. In mammals, the presequence is cleaved in a single step by the core proteins 1 and 2, which are related to the general mitochondrial matrix processing protease (MPP) a and (3 subunits the bovine heart presequence is retained as a 8.0 kDa subunit of the complex (42, 107). In Saccharomyces cerevis-iae, processing occurs in two steps Initially, the yeast MPP removes 22 amino acid residues to convert the precursor to the intermediate form, and then the mitochondrial intermediate protease (MIP) removes 8 residues after the intermediate form is in the bci complex (47). Cleavage by MIP is independent of the assembly of the Rieske cluster Conversion of the intermediate to the mature form was observed in a yeast mutant that did not assemble any Rieske cluster (35). However, in most mutants where the assembly of the Rieske cluster is prevented, the amount of Rieske protein is drastically reduced, most likely because of instability (35, 44). 

Nurse There are yeast mutants that capture the astral microtubules and orient the nucleus. It may be interesting to look at homologues of these. 

Nurse In fission yeast we can get asymmetrical divisions and make cells different sizes in various ways. Nuclear volume is influenced by subsequent cytoplasmic volume, and of course it is also directly influenced by ploidy. In an asymmetric division there will be a small nucleus and a big nucleus. 

The distribution of elements in single-cell non-photosynthetic eukaryotes is probably best seen in terms of the well-defined compartments of yeast. The central cytoplasmic compartment containing the nucleus has many free element concentrations, only somewhat different from those in all known aerobic prokaryotes (Figure 7.7). (The nuclear membrane is a poor barrier to small molecules and ions and so we include the nucleus with the cytoplasm.) We do not believe in fact that the free cytoplasmic values of Mg2+, Mn2+, Fe2+, Ca2+, and possibly Zn2+, have changed greatly throughout evolution. As stressed already there are limitations since free Mg2+ and Fe2+ are essential for the maintenance of the primary synthetic routes of all cells, and changes in other free metal ions could well have imposed... 

Bacteria normally harbour a single, circular chromosome that tends to be tethered to the bacterial plasma membrane and tends to have few if any closely associated proteins. Many bacteria also contain extra-chromosomal DNA in the form of plasmids, as will be discussed later. Eukaryotes (plants, animals and yeasts) posses multiple linear chromosomes contained within a cell nucleus, and these chromosomes are normally closely associated with proteins termed histones (the pro-tein-DNA complex is termed chromatin). Eukaryotes also invariably possess DNA sequences within mitochondria and in chloroplasts in plants. The (usually circular) DNA molecules are much... 

Adenylate cyclase was identified as the primary Ras target in yeast (Saccha-romyces cerevisiae) [56] but it took a while before in 1993 several groups independently found Raf to be the effector of Ras in mammals [41-44]. Shortly afterwards it was realized that this is not the only target of Ras but up until now it appears to be the most prominent one. Raf is a Ser/Thr-specific protein kinase which phosphorylates and thereby activates Mek which in turn phosphorylates and activates Erk, leading to an amplification of the signal. Erk, also termed MAPK, has a plethora of phosphorylation targets, the most important of which are transcription factors such as Elk-1, leading to activation of the transcription machinery in the nucleus. 

Figure 5. (Continued) different panels (e and f for HeLa, j and k for chicken erythrocyte, and o and p for yeast). A section profile obtained along X-Y line shows a typical granular structure in die nucleus (e, j, o), and the peak-to-peak distance between the granular structure was distributed from 60 nm to 120 nm (e). The diickness of the chromatin fibers released out of die nucleus varied possibly due to the assembly of diinner fibers (f, k, p). A section profile for the spread fibers was obtained along X-Y line (f, k, p). Isolated HeLa cell nucleus was treated widi (r, s) or without (q) RNase. The treatment releases SOnmfiber from the nucleus. The histogram of die fiber width is shown in an inset of (s). Bars, 250 nm. (See Colour Plate 2.)...

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