Maternal histones

In contrast to Xenopus laevis, the maternal histone pool in the mouse one-cell embryo (based on synthetic rates for histones) is probably sufficient for only one to two rounds of DNA replication (Wassarman and Mrozak, 1981). Consistent with such a small histone pool is the observation that poly spermic eggs have the capacity to transform up to three to four sperm nuclei into metaphase chromosomes (Clarke and Masui, 1986) a similar capacity was also determined from experiments that manipulated the cytoplasmic volume by either bisection or cell fusion (Clarke and Masui, 1987). This small pool of maternal histones may hence be insufficient to prevent effectively the assembly of stable basal transcription complexes. Thus, titration of the maternal histone pool by an increase in the mass of DNA due to blasto-mere proliferation may not be a critical factor in regulating the onset of transcription in the mouse embryo and other mammalian eggs this is because the maternal transcription factors may be able to outcompete successfully maternal histones for the newly replicated chromatin. This could, at least in part, account for the early onset of transcription in mammalian embryos ranging from rodents to humans (Telford et al., 1990). Moreover, the lack of arapid S phase in the mouse embryo and other mammalian embryos would permit sufficient time for productively assembled transcription complexes to generate full-length transcripts. In contrast to mammalian embryos, S phase is very short prior to the midblastula transition in Xenopus laevis (Newport and Kirschner, 1982) and hence these rapid rounds of DNA replication could prematurely terminate the transcription of genes for which transcription had initiated. 

More recent studies in Drosophila revealed that H3.3 also plays an important role in the male pronucleus after fertilization (Loppin et al. 2005). The loss of the H3.3 chaperone HIRA impairs the replacement of paternal non-histone proteins from the sperm nucleus with maternally provided histones including H3.3, while the maternal genome exclusively contains the canonical H3. Thus, H3.3 and its deposition factor HIRA function in early fertilization events and might have a role in imprinting in higher eukaryotes. 

Mandl, B., Brandt, W.F., Superti-Furga, G., Gratringer, P.G., Birnstiel, M.L., and Busslinger, M. (1997) The five cleavage-stage (CS) histones of the sea urchin are encoded by a maternally expressed... 

ADP-ribosylation has also been implicated as a proteolytic antagonist during embryonic development [231]. Following fertilization in sea urchin, sperm-specific histones are degraded by the sperm-histone-selective (SpH) protease and subsequently replaced by cleavage stage histone variants. During this process, the maternal replacement histones are protected from proteolysis by ADP-ribosylation. 

Genomic imprinting is the mechanism by, which only one of the two parental gene copies either maternal or paternal—is expressed. Modifications in DNA and histones are the marks of genomic imprinting. There are about 50 known imprinted genes in the human genome (64). For example, the... 

This protamine-histone exchange could provide a window of opportunity for maternally derived transcription factors to gain access to their c -cognate... 

Shortly after pronucleus formation and entry into S phase, transcription initiates in both pronuclei of the one-cell embryo (Figure 6). The male pronucleus supports higher levels of transcription than the female pronucleus, and this difference is attributed to their differences in chromatin structure that reflects their different histories (e.g., the protamine-histone exchange that occurs in the male pronucleus affords an opportunity for the sequestration of maternal transcription factors that is not available to the female pronucleus whose DNA is already packaged into chromatin). The first... 

Typically, 1.5 pg plasmid DNA is incubated with embryo extract (3 mg protein) containing 30 mM creatine phosphate, 3 mM MgCl2,3 mM ATP (pH 8.0) 0.1 pg/ ml creatine phosphokinase (type 1, Sigma), 1 mM DTT, and EX buffer to a total volume of 200 /u.1. The conductivity of the assembly reaction mix is equivalent to 65 mM KCl. The reaction is carried out for up to 6 hr at 26°C. Preblastoderm embryo extracts contain a maternal stockpile of core histones that are utilized in the assembly reaction. The linker histone, HI, on the other hand, is absent from the early embryo and, hence, from the reconstituted chromatin. However, exogenous HI can be incorporated into chromatin during the assembly reaction, where it increases the nucleosome repeat length from 180 to 197 bp DNA (Becker and Wu, 1992). 

Both histone and nonhistone proteins appear to be synthesized in the cytoplasm of sea urchin embryos, partly from maternal templates, and subsequently migrate to the cleavage nuclei (Kedes et al., 1969 Crane and Villee, 1971 Johnson and Hnilica, 1971 Moav and Nemer, 1971 Seale and Aronson, 1973a,b). Similar studies on amphibians have been scarce. The only direct analysis of nuclear proteins in amphibians is that reported by Asao (1969, 1972). Using a mixture of C-labeled amino acids, either injected into Triturus females during ovulation or... 

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