Immunological identity of proteins that bind stored 5S RNA in Xenopus oocytes

Histone H4 acetylation and transcription in amphibian chromatin

Lampbrush chromosomes from oocytes of the amphibian Triturus cristatus have been used to examine the role of histone acetylation in transcription by indirect immunofluorescence with antisera to H4 acetylated at specific lysine residues. Electrophoresis on acid-urea-Triton gels and Western blotting have confirmed the specificity of these antisera and defined the order in which particular lysine residues are acetylated in amphibian cells. As in mammals, lysine 16 is acetylated first, followed by 8 and/or 12 and then 5. With lampbrush chromosomes from immature (previtellogenic) oocytes, antisera to H4 acetylated at lysines 8, 12, and 16 labeled fluorescent foci at the bases of transcription loops. Antisera to H4 acetylated at lysine 5 labeled weakly (i.e., the tri- and tetraacetylated isoforms must be rare). Loops showed weak labeling of the chromatin axis but intense fluorescence at particular points, which probably represent incompletely decondensed chromatin. The RNP matrix of loops, including the RNP-rich sphere bodies and the dense matrix of "marker" loops, was not labeled. Treatment of immature oocytes with butyrate for 12 h to inhibit histone deacetylation did not affect immunolabeling, suggesting that turnover of H4 acetates is slow. In contrast, in chromosomes from mature oocytes, in which loops have retracted and transcription is low, butyrate caused an increase in labeling with all antisera, followed by the appearance of vestigial loops, weakly labeled, but with regions of intense fluorescence. These loops contain RNP and are presumably transcriptionally active. We conclude that H4 acetates turn over more rapidly in mature than immature oocytes and that histone hyperacetylation precedes, and possibly induces, loop formation and transcriptional activation.

Physical and immunological characterization of human transcription factor IIIA

Human transcription factor IIIA (htFIIIA), specifically required for transcription of the gene for 5S ribosomal RNA has been characterized with respect to some of its physical, immunological and functional properties. TFIIIA from HeLa cells, which selectively binds 5S RNA, is a monomer of approximately 35 kDa with a Stokes' radius of approximately 2.65 nm and a sedimentation coefficient of approximately 2.8 S. These values indicate that the human protein is of rather globular shape and hence diverges not only in molecular mass but also in most of the molecular properties from its highly asymmetric counterpart in Xenopus laevis oocytes. By raising specific polyclonal antibodies against hTFIIIA it was shown in Western immunoblots that there was no cross-reaction between anti-hTFIIIA antibodies and the amphibian protein. Conversely, monoclonal antibodies against three domains of X. laevis TFIIIA antibodies and the amphibian protein. Conversely, monoclonal antibodies against three domains of X. laevis TFIIIA did not cross-react with the human transcription factor. The polyclonal antisera raised against hTFIIIA specifically neutralized binding of the human transcription factor to 5S DNA and abolished in vitro transcription of 5S RNA but these antibodies were unable to inhibit 5S RNA synthesis in cellular extracts from Xenopus, Drosophila or yeast cells. Finally, the species variation of TFIIIA could be substantiated by electrophoretic mobility shift assays revealing preferential binding of hTFIIIA to the homologous 5S RNA gene.

An alternative protein factor which binds the internal promoter of Xenopus 5S ribosomal RNA genes

In small oocytes of Xenopus species, two sets of 5S RNA genes, oocyte-type and somatic-type, are fully activated. The 5S RNA transcripts are temporarily stored, half in association with TFIIIA to form a 7S particle, the other half in association with tRNA and two proteins (p48 and p43) to form a 42S particle. It has been established previously that TFIIIA binds to the internal control region of 5S RNA genes and promotes their transcription. Here we show that protein can be translocated from the 42S particles to 5S RNA genes, but only after treatment of the particles with ribonuclease. Nevertheless, once transferred, stable protein-DNA complexes are formed and DNase-protection experiments show that binding is specific to the gene promoter, covering exactly the same sequence as TFIIIA. The DNA-binding protein is identified as p48 which, after isolation by ion-exchange chromatography, will bind to 5S RNA genes in the absence of ribonuclease.

Identification of a 60-kDa phosphoprotein that binds stored messenger RNA of Xenopus oocytes

Rapidly labelled, polyadenylated RNA is contained in three distinct fractions isolated from homogenized amphibian oocytes: (a) in ribonucleoprotein particles that are associated with a fibrillar matrix, the complexes sedimenting at greater than 1500S; (b) in ribonucleoprotein particles that sediment at 20-120S and have the characteristics of stored (maternal) messenger ribonucleoprotein (mRNP) and (c) in polyribosomes that sediment at 120-360S. We have compared the RNA and protein components of the first two of these RNP fractions. The polyadenylated RNA extracted from the two RNP fractions differs in that the RNA from fibril-associated RNP contains a much higher content of repeat sequences than does the RNA from mRNP. In other words, the RNA from fibril-associated RNP is largely unprocessed and may constitute a premessenger state, which for convenience is referred to as premessenger RNP (pre-mRNP). RNA-binding experiments demonstrate that the polypeptide most tightly bound in pre-mRNP is a 54-kDa component (p54), whereas the polypeptide most tightly bound in mRNP is a 60-kDa component (p60). Antibodies raised against p60 are used to show that this polypeptide is a common major component of pre-mRNP and mRNP and that it is also located in oocyte nuclei. However the state of p60 is modified between the premessenger and stored message levels: the polypeptide in mRNP is heavily phosphorylated whereas the equivalent polypeptide in pre-mRNP is completely unphosphorylated. The relative roles of the presence of repeat sequences and phosphorylation of mRNA-associated protein in blocking translation are discussed.

Zinc ions are differentially required for the transcription of ribosomal 5S RNA and tRNA in a HeLa-cell extract

Chelation of divalent cations by 5 mM EDTA and subsequent removal by dialysis from a cytoplasmic HeLa cell extract leads to a complete loss of 5S rRNA transcription without affecting tRNA synthesis. Transcription complexes for 5S RNA can no longer be assembled in such a zinc-depleted extract and this ability can be fully restored only by the re-addition of 5 microM zinc. Reconstitution experiments with isolated protein fractions show that transcription factor A from HeLa-cells requires zinc to exert its specific function. Pre-formation of transcription complexes partially protects the metal ion against removal by chelation even in the presence of 1.8 M KCl. These results indicate that the zinc ions are bound to mammalian transcription factor IIIA which, in a transcription complex, binds very strongly to the 5S RNA gene. Cation depletion with 75 mM EDTA also suppresses tRNA transcription; an effect which is reversible by zinc addition. We conclude that beside for the binding of TF IIIA, zinc is also bound with a different affinity to a transcription component common to 5S and tRNA synthesis, most likely polymerase III itself.

Distribution and utilization of 5 S-RNA-binding proteins during the development of Xenopus oocytes

At early stages of oogenesis in Xenopus laevis most of the ribosomal 5S RNA is complexed with three proteins to form two types of cytoplasmic RNP storage particle. A particle sedimenting at 42S contains 5S RNA and tRNA together with two proteins of Mr 48000 (P48) and Mr 43000 (P43) and a second particle sedimenting at 7S contains 5S RNA plus a protein of Mr 40000 (P40, also known as the transcription factor, TFIIIA). In this report we use antibodies monospecific for each protein to follow the movement of 5S RNA from nucleus to cytoplasm to nucleolus to cytoplasm and to determine the fate of each of the proteins that associate with 5S RNA during these transitions. Both P48 and P43 have roles additional to the formation of the 42S RNP storage particle; P48 is detected in the nucleus during early oogenesis and is cleaved to yield an Mr-33000 fragment that remains associated with 5S RNA that is excess to ribosome requirement during late oogenesis; P43 appears to be cleaved to yield fragments of Mr 28000 and 17000, the latter being present in ribosomal fractions. Apparently, there is no function for P40 in addition to those already described in transcription of 5S RNA genes and in storage of 5S RNA as a 7S RNP particle.