The isolation and characterization of a second oocyte 5s DNA from Xenopus laevis

A second and minor DNA component containing 5s RNA genes has been purified from the genomic DNA of Xenopus laevis (XIt 5S DNA). Some of its physical and chemical characteristics are described. It contains a 5S RNA gene sequence which has some oocyte and some somatic-specific residues, as well as nucleotides which differ from both types of 5S RNA. There are about 2000 of these 5S RNA genes per haploid complement of DNA compared to about 24,000 of the principal oocyte 5S RNA genes. The multiple repeating units of XIt 5S DNA are homogeneous in length (about 350 base pairs). We present evidence that XIt 5S RNA is transcribed in ovaries but not in somatic cells; XIt 5S DNA may therefore be under the same control as the major oocyte 5S DNA.     

Assembly of transcriptionally active chromatin in Xenopus oocytes requires specific DNA binding factors

Active minichromosomes assembled on injected 5S RNA gene clones are stable in Xenopus oocytes; endogenous 5S DNA specific factor(s) are required for their assembly. When somatic-type and oocyte-type 5S RNA gene clones are coinjected, the somatic genes are assembled into active minichromosomes, while most of the oocyte genes are assembled into inactive ones. The differential 5S RNA gene expression, which mimics that in somatic cells, appears to result from titration of 5S DNA specific factor(s) by the competing somatic 5S DNA, followed by histone mediated assembly of inactive chromatin on the oocyte 5S DNA. Stable minichromosomes are also assembled on a cloned histone H4 gene; again, intragenic DNA rearrangements affect the efficiency of assembly of active chromatin and differential gene expression occurs after coinjection of two or more H4 DNA constructs. We suggest that the H4 DNA molecules also compete for limiting quantities of specific DNA binding factor(s) required for the assembly of active H4 gene chromatin.     

Differential gene expression in fully-grown oocytes between gynogenetic and gonochoristic crucian carps.

Silver crucian carp (Carassius auratus gibelio) is a unique triploid bisexual species that can reproduce by gynogenesis. As all other gynogenetic animals, it keeps its chromosome integrity by inhibiting the first meiosis division (no extrusion of the first pole body). To understand the molecular events governing this reproduction mode, suppression subtractive hybridization was used to identify the genes differentially expressed in fully-grown oocytes of the gynogenetic and gonochoristic crucian carp (gyno-carp and gono-carp). From two specific subtractive cDNA libraries, the clones screened out by dot blots and virtual Northern blots were chosen to clone full-length cDNA by RACE. Four differentially expressed genes were obtained. Two are novel genes and are expressed specifically in the oocytes. The gyno-carp stores much more mRNA of cyclin A2, a new member of the fish A-type cyclin gene, in its fully-grown oocyte than in the gono-carp. The last gene is histone H2A. The histone H2As of these two closely related crucian carps are quite different in the C-terminus. Preliminary characterization of the four genes has been analyzed by nucleotide and deduced amino acid sequence and Northern analysis.     

The structural organization and DNA sequence of a wheat histone H4 gene.

Some wheat histone H4 genes have been cloned from a Charon 4 wheat genomic DNA library using sea urchin histone H4 DNA as a probe. DNA sequence analysis of a cloned gene showed that the deduced amino acid sequence of wheat histone H4 protein was identical to that of pea. The 5' end of wheat histone H4 mRNA was mapped on the cloned gene by the S1-procedure. Southern blotting analysis of the genomic DNA indicated that histone H4 genes were reiterated 100 to 125 times per hexaploid wheat genome.     

Protein migration into nuclei. I. Frog oocyte nuclei in vivo accumulate microinjected histones, allow entry to small proteins, and exclude large proteins

A technique is presented which enables one to measure the extent to which a protein enters and accumulates in the nucleus of the frog oocyte. In this method, the protein, labeled with 125-I, is microinjected into the oocyte. After incubation, the oocyte is manually enucleated and the radioactivity in the nucleus and cytoplasm is determined. Using this technique, proteins lighter than 20,000 daltons were found to enter the nucleus and completely equilibrate between the nucleus and cytoplasm within 24 h. The entry of proteins heavier than 69,000 daltons was severely hindered. Histones and histone fractions entered as quickly as other small proteins, but, in contrast to these proteins, they accumulated in the nucleus to different extents, depending on the total amount of histone injected into the oocyte and the identity of the histone. Evidence is presented that histone fractions compete with each other for accumulation in the nucleus.     

Two discrete modes of histone gene expression during oogenesis in Drosophila melanogaster

We have used in situ hybridization to ovarian tissue sections to study the pattern of histone gene expression during oogenesis in Drosophila melanogaster. Our studies suggest that there are two distinct phases of histone gene expression during oogenesis. In the first phase, which occurs during early to middle oogenesis (stages 5-10A), we observe a mosaic pattern of histone mRNA in the 15 nurse cells of the egg chamber: some cells have very high levels of mRNA, while others have little or no mRNA. Our analysis suggests that there is a cyclic accumulation and subsequent degradation of histone mRNA in the egg chamber and that very little histone mRNA is transported into the growing oocyte. Moreover, since the endomitotic replication cycles of the nurse cells are asynchronous during this period, the mosaic distribution of histone message would suggest that the expression of the histone genes in each nurse cell nucleus is probably coupled to DNA replication as in most somatic cells. The second phase begins at stage 10B. During this period, histone gene expression appears to be "induced" in all 15 nurse cells of the egg chamber, and instead of a mosaic pattern, high levels of histone mRNA are found in all cells. Unlike the earlier phase, this expression is apparently uncoupled from the endomitotic replication of the nurse cells (which are completed by the end of stage 10A). Moreover, much of the newly synthesized histone mRNA is transported from the nurse cells into the oocyte where it accumulates and is stored for use during early embryogenesis. Finally, we have also observed tightly clustered grains within nurse cell nuclei in non-denatured tissue sections. As was the case with cytoplasmic histone mRNA, there is a mosaic distribution of nuclear grains from stages 5 to 10A, while at stage 10B, virtually all nurse cell nuclei have grain clusters. These grain clusters appear to be due to the hybridization of nurse cell histone gene DNA to our probe, and are localized in specific regions of the nucleus.     

Xenopus nucleoplasmin: egg vs. oocyte

Nucleoplasmin has been purified from either oocytes or unfertilized eggs of the frog, Xenopus laevis. We find that the pentameric form of egg nucleoplasmin exhibits an apparent molecular mass approximately 15 000 daltons larger than its oocyte counterpart upon sodium dodecyl sulfate (SDS)-acrylamide gel electrophoresis. Egg nucleoplasmin monomers are more heterogeneous, substantially more acidic, and overall larger in apparent molecular weight than oocyte nucleoplasmin monomers when analyzed by isoelectric focusing or SDS gel electrophoresis. Protease digestions indicate that the structural differences between egg and oocyte nucleoplasmin are primarily confined to the N-terminal halves of the proteins. The structural diversity observed is accompanied by a difference in the ability of nucleoplasmin from the two sources to act as a nucleosome assembly agent in vitro. Egg nucleoplasmin efficiently promotes the formation of nucleosomes onto circular pBR322 DNA in vitro at physiological ionic strength and at physiological histone:DNA ratios, while oocyte nucleoplasmin is markedly deficient in serving as an in vitro chromatin assembly agent under all conditions which we have tested. Treatment of egg nucleoplasmin in vitro with alkaline phosphatase demonstrates that the structural diversity between egg and oocyte nucleoplasmin results primarily from extensive additional phosphorylation of the egg protein. The relevance of nucleoplasmin phosphorylation in leading to differences in the chromatin assembly activity of this protein both in vitro and in vivo is considered.