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(1/6057) Meiosis: MeiRNA hits the spot.

The protein Mei2 performs at least two functions required in fission yeast for the switch from mitotic to meiotic cell cycles. One of these functions also requires meiRNA. It appears that meiRNA targets Mei2 to the nucleus, where it can promote the first meiotic division.  (+info)

(2/6057) SWM1, a developmentally regulated gene, is required for spore wall assembly in Saccharomyces cerevisiae.

Meiosis in Saccharomyces cerevisiae is followed by encapsulation of haploid nuclei within multilayered spore walls. Formation of this spore-specific wall requires the coordinated activity of enzymes involved in the biosynthesis of its components. Completion of late events in the sporulation program, leading to spore wall formation, requires the SWM1 gene. SWM1 is expressed at low levels during vegetative growth but its transcription is strongly induced under sporulating conditions, with kinetics similar to those of middle sporulation-specific genes. Homozygous swm1Delta diploids proceed normally through both meiotic divisions but fail to produce mature asci. Consistent with this finding, swm1Delta mutant asci display enhanced sensitivity to enzymatic digestion and heat shock. Deletion of SWM1 specifically affects the expression of mid-late and late sporulation-specific genes. All of the phenotypes observed are similar to those found for the deletion of SPS1 or SMK1, two putative components of a sporulation-specific MAP kinase cascade. However, epistasis analyses indicate that Swm1p does not form part of the Sps1p-Smk1p-MAP kinase pathway. We propose that Swm1p, a nuclear protein, would participate in a different signal transduction pathway that is also required for the coordination of the biochemical and morphological events occurring during the last phase of the sporulation program.  (+info)

(3/6057) Comparative sequence analysis of human minisatellites showing meiotic repeat instability.

The highly variable human minisatellites MS32 (D1S8), MS31A (D7S21), and CEB1 (D2S90) all show recombination-based repeat instability restricted to the germline. Mutation usually results in polar interallelic conversion or occasionally in crossovers, which, at MS32 at least, extend into DNA flanking the repeat array, defining a localized recombination hotspot and suggesting that cis-acting elements in flanking DNA can influence repeat instability. Therefore, comparative sequence analysis was performed to search for common flanking elements associated with these unstable loci. All three minisatellites are located in GC-rich DNA abundant in dispersed and tandem repetitive elements. There were no significant sequence similarities between different loci upstream of the unstable end of the repeat array. Only one of the three loci showed clear evidence for putative coding sequences near the minisatellite. No consistent patterns of thermal stability or DNA secondary structure were shared by DNA flanking these loci. This work extends previous data on the genomic environment of minisatellites. In addition, this work suggests that recombinational activity is not controlled by primary or secondary characteristics of the DNA sequence flanking the repeat array and is not obviously associated with gene promoters as seen in yeast.  (+info)

(4/6057) hMSH5: a human MutS homologue that forms a novel heterodimer with hMSH4 and is expressed during spermatogenesis.

MutS homologues have been identified in nearly all organisms examined to date. They play essential roles in maintaining mitotic genetic fidelity and meiotic segregation fidelity. MutS homologues appear to function as a molecular switch that signals genomic manipulation events. Here we describe the identification of the human homologue of the Saccharomyces cerevisiae MSH5, which is known to participate in meiotic segregation fidelity and crossing-over. The human MSH5 (hMSH5) was localized to chromosome 6p22-21 and appears to play a role in meiosis because expression is induced during spermatogenesis between the late primary spermatocytes and the elongated spermatid phase. hMSH5 interacts specifically with hMSH4, confirming the generality of functional heterodimeric interactions in the eukaryotic MutS homologue, which also includes hMSH2-hMSH3 and hMSH2-hMSH6.  (+info)

(5/6057) Sequential PKC- and Cdc2-mediated phosphorylation events elicit zebrafish nuclear envelope disassembly.

Molecular markers of the zebrafish inner nuclear membrane (NEP55) and nuclear lamina (L68) were identified, partially characterized and used to demonstrate that disassembly of the zebrafish nuclear envelope requires sequential phosphorylation events by first PKC, then Cdc2 kinase. NEP55 and L68 are immunologically and functionally related to human LAP2beta and lamin B, respectively. Exposure of zebrafish nuclei to meiotic cytosol elicits rapid phosphorylation of NEP55 and L68, and disassembly of both proteins. L68 phosphorylation is completely inhibited by simultaneous inhibition of Cdc2 and PKC and only partially blocked by inhibition of either kinase. NEP55 phosphorylation is completely prevented by inhibition or immunodepletion of cytosolic Cdc2. Inhibition of cAMP-dependent kinase, MEK or CaM kinase II does not affect NEP55 or L68 phosphorylation. In vitro, nuclear envelope disassembly requires phosphorylation of NEP55 and L68 by both mammalian PKC and Cdc2. Inhibition of either kinase is sufficient to abolish NE disassembly. Furthermore, novel two-step phosphorylation assays in cytosol and in vitro indicate that PKC-mediated phosphorylation of L68 prior to Cdc2-mediated phosphorylation of L68 and NEP55 is essential to elicit nuclear envelope breakdown. Phosphorylation elicited by Cdc2 prior to PKC prevents nuclear envelope disassembly even though NEP55 is phosphorylated. The results indicate that sequential phosphorylation events elicited by PKC, followed by Cdc2, are required for zebrafish nuclear disassembly. They also argue that phosphorylation of inner nuclear membrane integral proteins is not sufficient to promote nuclear envelope breakdown, and suggest a multiple-level regulation of disassembly of nuclear envelope components during meiosis and at mitosis.  (+info)

(6/6057) Characterization of a Caenorhabditis elegans recA-like gene Ce-rdh-1 involved in meiotic recombination.

A recA-like gene was identified in the Caenorhabditis elegans genome project database. The putative product of the gene, termed Ce-rdh-1 (C. elegans RAD51 and DMC1/LIM15 homolog 1), consists of 357 amino acid residues. The predicted amino acid sequence of Ce-rdh-1 showed 46-60% identity to both RAD51 type and DMC1/LIM15 type genes in several eukaryote species. The results of RNAi (RNA-mediated interference) indicated that repression of Ce-rdh-1 blocked chromosome condensation of six bivalents and dissociation of chiasmata in oocytes of F1 progeny. Oogenesis did not proceed to the diakinesis stage. Accordingly, all the eggs produced (F2) died in early stages. These results suggest that Ce-rdh-1 participates in meiotic recombination.  (+info)

(7/6057) Gene expression and chromatin organization during mouse oocyte growth.

Mouse oocytes can be classified according to their chromatin organization and the presence [surrounded nucleolus (SN) oocytes] or absence [nonsurrounded nucleolus (NSN) oocytes] of a ring of Hoechst-positive chromatin around the nucleolus. Following fertilization only SN oocytes are able to develop beyond the two-cell stage. These studies indicate a correlation between SN and NSN chromatin organization and the developmental competence of the female gamete, which may depend on gene expression. In the present study, we have used the HSP70.1Luc transgene (murine HSP70.1 promoter + reporter gene firefly luciferase) to analyze gene expression in oocytes isolated from ovaries of 2-day- to 13-week-old females. Luciferase was assayed on oocytes after classification as SN or NSN type. Our data show that SN oocytes always exhibit a higher level of luciferase activity, demonstrating a higher gene expression in this category. Only after meiotic resumption, metaphase II oocytes derived from NSN or SN oocytes acquire the same level of transgene expression. We suggest that the limited availability of transcripts and corresponding proteins, excluded from the cytoplasm until GVBD in NSN oocytes, could explain why these oocytes have a lower ability to sustain embryonic development beyond the two-cell stage at which major zygotic transcription occurs. With this study we have furthered our knowledge of epigenetic regulation of gene expression in oogenesis.  (+info)

(8/6057) Germ cell development in the XXY mouse: evidence that X chromosome reactivation is independent of sexual differentiation.

Prior to entry into meiosis, XX germ cells in the fetal ovary undergo X chromosome reactivation. The signal for reactivation is thought to emanate from the genital ridge, but it is unclear whether it is specific to the developing ovary. To determine whether the signals are present in the developing testis as well as the ovary, we examined the expression of X-linked genes in germ cells from XXY male mice. To facilitate this analysis, we generated XXY and XX fetuses carrying X chromosomes that were differentially marked and subject to nonrandom inactivation. This pattern of nonrandom inactivation was maintained in somatic cells but, in XX as well as XXY fetuses, both parental alleles were expressed in germ cell-enriched cell populations. Because testis differentiation is temporally and morphologically normal in the XXY testis and because all germ cells embark upon a male pathway of development, these results provide compelling evidence that X chromosome reactivation in fetal germ cells is independent of the somatic events of sexual differentiation. Proper X chromosome dosage is essential for the normal fertility of male mammals, and abnormalities in germ cell development are apparent in the XXY testis within several days of X reactivation. Studies of exceptional germ cells that survive in the postnatal XXY testis demonstrated that surviving germ cells are exclusively XY and result from rare nondisjunctional events that give rise to clones of XY cells.  (+info)