Genes located in chromosomal regions near telomeres are transcriptionally silent, whereas those located in regions away from telomeres are not. Here we show that there is a gradient of acetylation of histone H4 at lysine 16 (H4-Lys16) along a yeast chromosome; this gradient ranges from a hypoacetylated state in regions near the telomere to a hyperacetylated state in more distant regions. The hyperacetylation is regulated by Sas2p, a member of the MYST-type family of histone acetylases, whereas hypoacetylation is under the control of Sir2p, a histone deacetylase. Loss of hyperacetylation is accompanied by an increase in localization of the telomere protein Sir3p and the inactivation of gene expression in telomere-distal regions. Thus, the Sas2p and Sir2p function in concert to regulate transcription in yeast, by acetylating and deacetylating H4-Lys16 in a mechanism that may be common to all eukaryotes.

Regulation of interkinetic nuclear migration by cell cycle-coupled active and passive mechanisms in the developing brainIn proliferating neural epithelia, cells undergo interkinetic nuclear migration: stereotyped cell cycle-dependent movements in the apico-basal plane. The microtubule-binding protein Tpx2 is here shown to regulate the G2-phase basal-to-apical migration, while passive displacement effects are responsible for basally directed movements.

A male pronucleus migrates toward the center of an egg to reach the female pronucleus for zygote formation. This migration depends on microtubules growing from two centrosomes associated with the male pronucleus. Two mechanisms were previously proposed for this migration: a "pushing mechanism," which uses the pushing force resulting from microtubule polymerization, and a "pulling mechanism," which uses the length-dependent pulling force generated by minus-end-directed motors anchored throughout the cytoplasm. We combined two computer-assisted analyses to examine the relative contribution of these mechanisms to male pronuclear migration. Computer simulation revealed an intrinsic difference in migration behavior of the male pronucleus between the pushing and pulling mechanisms. In vivo measurements using image processing showed that the actual migration behavior in Caenorhabditis elegans confirms the pulling mechanism. A male pronucleus having a single centrosome migrated toward the single aster. We propose that the pulling mechanism is the primary mechanism for male pronuclear migration.

No abstract

Background: Tip60, an HIV-1-Tat interactive protein, is a nuclear histone acetyltransferase (HAT) with unique histone substrate specificity. Since the acetylation of core histones at particular lysines mediates distinct effects on chromatin assembly and gene regulation, the identification of lysine site specificity of the HAT activity of Tip60 is an initial step in the analysis of its molecular function.

The centrosome is generally maintained at the center of the cell. In animal cells, centrosome centration is powered by the pulling force of microtubules, which is dependent on cytoplasmic dynein. However, it is unclear how dynein brings the centrosome to the cell center, i.e., which structure inside the cell functions as a substrate to anchor dynein. Here, we provide evidence that a population of dynein, which is located on intracellular organelles and is responsible for organelle transport toward the centrosome, generates the force required for centrosome centration in Caenorhabditis elegans embryos. By using the database of full-genome RNAi in C. elegans, we identified dyrb-1, a dynein light chain subunit, as a potential subunit involved in dynein anchoring for centrosome centration. DYRB-1 is required for organelle movement toward the minus end of the microtubules. The temporal correlation between centrosome centration and the net movement of organelle transport was found to be significant. Centrosome centration was impaired when Rab7 and RILP, which mediate the association between organelles and dynein in mammalian cells, were knocked down. These results indicate that minus end-directed transport of intracellular organelles along the microtubules is required for centrosome centration in C. elegans embryos. On the basis of this finding, we propose a model in which the reaction forces of organelle transport generated along microtubules act as a driving force that pulls the centrosomes toward the cell center. This is the first model, to our knowledge, providing a mechanical basis for cytoplasmic pulling force for centrosome centration.endosome | lysosome | pronuclear migration | the centrosome-organelle mutual pulling model | yolk granule

Cell size is one of the critical parameters controlling the size of intracellular structures. A well-known example is the constant nuclear-to-cytoplasmic ratio (N/C ratio) [1-5]. The length of the metaphase spindle is proportional to cell size, but it has an upper limit during early embryogenesis [6]. During anaphase, the mitotic spindle elongates and delivers the centrosomes and sister chromatids near the centers of the nascent daughter cells. Here, we quantified the relationship between spindle elongation and cell size in the early embryo of Caenorhabditis elegans and propose possible models for cell-size-dependent spindle elongation. Quantitative measurements revealed that the extent and speed of spindle elongation are correlated with cell size throughout early embryogenesis. RNAi knockdown of Galpha proteins and their regulators revealed that the spindles failed to fully elongate and that the speed of spindle elongation was almost constant regardless of cell size. Our results suggest that spindle elongation is controlled by two qualitatively distinct mechanisms, i.e., Galpha-dependent and -independent modes of elongation. Simulation analyses revealed that the constant-pulling model and the force-generator-limited model reproduced the dynamics of the Galpha-independent and Galpha-dependent mechanisms, respectively. These models also explain how the set length of spindles is achieved.

To faithfully target the center of large eggs, microtubule asters translate their shape into directed motion through length-dependent microtubule forces mediated by dynein in the cytoplasm. Their speed is independent of aster shape but determined by their growth rate.