Somatic-type 5S RNA Research Articles

The Core Histone Tail Domains Contribute to Sequence-dependent Nucleosome Positioning

The precise positioning of nucleosomes plays a critical role in the regulation of gene expression by modulating the DNA binding activity of trans-acting factors. However, molecular determinants responsible for positioning are not well understood. We examined whether the removal of the core histone tail domains from nucleosomes reconstituted with specific DNA fragments led to alteration of translational positions. Remarkably, we find that removal of tail domains from a nucleosome assembled on a DNA fragment containing a Xenopus borealis somatic-type 5S RNA gene results in repositioning of nucleosomes along the DNA, including two related major translational positions that move about 20 bp further upstream with respect to the 5S gene. In a nucleosome reconstituted with a DNA fragment containing the promoter of a Drosophila alcohol dehydrogenase gene, several translational positions shifted by about 10 bp along the DNA upon tail removal. However, the positions of nucleosomes assembled with a DNA fragment known to have one of the highest binding affinities for core histone proteins in the mouse genome were not altered by removal of core histone tail domains. Our data support the notion that the basic tail domains bind to nucleosomal DNA and influence the selection of the translational position of nucleosomes and that once tails are removed movement between translational positions occurs in a facile manner on some sequences. However, the effect of the N-terminal tails on the positioning and movement of a nucleosome appears to be dependent on the DNA sequence such that the contribution of the tails can be masked by very high affinity DNA sequences. Our results suggest a mechanism whereby sequence-dependent nucleosome positioning can be specifically altered by regulated changes in histone tail-DNA interactions in chromatin.

Nucleolar targeting of 5S RNA in Xenopus laevis oocytes: somatic-type nucleotide substitutions enhance nucleolar localization.

In Xenopus laevis oocytes, 5S RNA is stored in the cytoplasm until vitellogenesis, at which time it is imported into the nucleus and targeted to nucleoli for ribosome assembly. This article shows that throughout oogenesis there is a pool of nuclear 5S RNA which is not nucleolar-associated. This distribution reflects that of oocyte-type 5S RNA, which is the major 5S RNA species in oocytes; only small amounts of somatic-type, which differs by six nucleotides, are synthesized. Indeed, 32P-labeled oocyte-type 5S RNA showed a degree of nucleolar localization similar to endogenous 5S RNA (33%) after microinjection. In contrast, 32P-labeled somatic-type 5S RNA showed significantly enhanced localization, whereby 70% of nuclear RNA was associated with nucleoli. A chimeric RNA molecule containing only one somatic-specific nucleotide substitution also showed enhanced localization, in addition to other somatic-specific phenotypes, including enhanced nuclear import and ribosome incorporation. The distribution of 35S-labeled ribosomal protein L5 was similar to that of oocyte-type 5S RNA, even when preassembled with somatic-type 5S RNA. The distribution of a series of 5S RNA mutants was also analyzed. These mutants showed various degrees of localization, suggesting that the efficiency of nucleolar targeting can be influenced by many discrete regions of the 5S RNA molecule.

The AT-rich flanks of the oocyte-type 5S RNA gene of Xenopus laevis act as a strong local signal for histone H1-mediated chromatin reorganization in vitro.

In vivo, histone H1 plays an active role in establishing the transcriptionally repressed chromatin state of the oocyte-type 5S RNA genes in the early stages of Xenopus development. By using fully defined in vitro system of chromatin assembly on plasmids with cloned oocyte- or somatic-type 5S gene repeats we found that the oocyte repeat which comprises a 120 bp oocyte-type 5S RNA gene placed within the few hundred bp long native AT-rich flanks, but not the somatic repeat (a similar 120 bp somatic-type 5S RNA gene placed within native GC-rich flanks) enables histone H1 to realign the nucleosomal core particles densely packed on plasmid DNA. The realignment results in creation of the repeat unit of approximately 240 bp and is achieved through complete removal of several core histone complexes from plasmid template with the oocyte-type repeat. This effect of H1 is independent on the plasmid sequences and seems to be solely due to the presence in the oocyte-repeat of the AT-rich flanks. The effects of H1 are completely suppressed by distamycin A, a drug that specifically recognizes and binds oligo(dA).oligo(dT) runs in DNA. The binding of H1 results in increased protection of DNA sites within the AT-rich oocyte-type 5S repeat. In an in vitro transcription assay performed with reconstituted chromatin templates containing plasmids with the oocyte- or somatic-type repeats only the transcription of the oocyte-type 5S RNA gene was repressed in the presence of physiological concentration of histone H1. These results support the view that the AT-rich flanks of the oocyte-type 5S RNA gene are involved in histone H1-mediated chromatin reorganization that results in the transcriptional repression observed in vivo.

Role of Maturation-Promoting Factor (p34cdc2-Cyclin B) in Differential Expression of the Xenopus Oocyte and Somatic-Type 5S RNA Genes

Transcription of 5S rRNA and tRNA genes by RNA polymerase III (pol III) in cytosolic extracts of unfertilized Xenopus eggs and in a reconstituted system derived from Xenopus oocytes is repressed by the action of one or more mitotic protein kinases. Repression is due to the phosphorylation of a component of the pol III transcription apparatus. We find that the maturation/mitosis-promoting factor kinase (MPF, p34cdc2-cyclin B) can directly mediate this repression in vitro. Affinity-purified MPF and immune complexes formed with antibodies to the protein subunits of MPF (p34cdc2 and cyclin B) retain both histone H1 kinase activity and the capacity to repress transcription in the reconstituted transcription system. Transcription complexes of oocyte-type 5S RNA genes and tRNA genes are quantitatively more sensitive to MPF repression than the corresponding transcription complexes of the somatic-type 5S RNA gene. The differential transcription of oocyte- and somatic-type genes observed during early Xenopus embryogenesis has been reproduced with the reconstituted transcription system and affinity-purified MPF. This differential transcription may be due to the instability of transcription complexes on the oocyte-type genes and the heightened sensitivity of soluble transcription factors to inactivation by mitotic phosphorylation. Our results suggest that MPF may play a role in vivo in the establishment of the embryonic pattern of pol III gene expression.

Role of maturation-promoting factor (p34cdc2-cyclin B) in differential expression of the Xenopus oocyte and somatic-type 5S RNA genes.

Transcription of 5S rRNA and tRNA genes by RNA polymerase III (pol III) in cytosolic extracts of unfertilized Xenopus eggs and in a reconstituted system derived from Xenopus oocytes is repressed by the action of one or more mitotic protein kinases. Repression is due to the phosphorylation of a component of the pol III transcription apparatus. We find that the maturation/mitosis-promoting factor kinase (MPF, p34cdc2-cyclin B) can directly mediate this repression in vitro. Affinity-purified MPF and immune complexes formed with antibodies to the protein subunits of MPF (p34cdc2 and cyclin B) retain both histone H1 kinase activity and the capacity to repress transcription in the reconstituted transcription system. Transcription complexes of oocyte-type 5S RNA genes and tRNA genes are quantitatively more sensitive to MPF repression than the corresponding transcription complexes of the somatic-type 5S RNA gene. The differential transcription of oocyte- and somatic-type genes observed during early Xenopus embryogenesis has been reproduced with the reconstituted transcription system and affinity-purified MPF. This differential transcription may be due to the instability of transcription complexes on the oocyte-type genes and the heightened sensitivity of soluble transcription factors to inactivation by mitotic phosphorylation. Our results suggest that MPF may play a role in vivo in the establishment of the embryonic pattern of pol III gene expression.

Contribution of Individual Base Pairs to the Interaction of TFIIIA with the Xenopus 5S RNA Gene

The effects of a series of point mutations within the Xenopus borealis somatic-type 5S RNA gene on transcription factor IIIA (TFIIIA) binding affinity were quantified. These data define a critical sequence-dependent contact region within the classical box C promoter element from base pair 80 to 91. Substitution of GC base pairs at positions 81, 85, 86, 89, and 91 significantly reduce TFIIIA binding affinity. Base pairs located at other positions within the box C contact region provide a moderate contribution to TFIIIA-5S gene interaction. In contrast to the extensive set of sequence contacts within the box C element, TFIIIA interaction is localized primarily to two GC base pairs at positions 70 and 71 within the intermediate promoter element. A selected amplification and binding assay (SAAB) was performed with a synthetic internal control region (ICR) randomized from base pair 78 to 95 to identify box C promoter sequences bound with high affinity by TFIIIA. The wild-type 5S RNA gene sequence from 79 to 92 is strongly selected. These results are consistent with the critical role of the box C element in sequence-dependent promoter recognition by TFIIIA.

Replication Timing and Xenopus 5S RNA Gene Transcription in Vitro

The influence of DNA replication in a chromospinal environment on the transcription of Xenopus laevis 5S RNA genes has been examined using low-speed extracts of Xenopus eggs and Xenopus sperm nuclei. The major oocyte- and somatic-type 5S RNA multigene families of X. laevis are differentially expressed in these extracts dependent on limiting transcription factors and replicate in different compartments of S-phase. Under these particular experimental conditions the timing of replication of the somatic 5S RNA genes appears to be independent of their transcriptional activity. Moreover, differential transcription of the oocyte- and somatic-type 5S RNA genes is not influenced by the replication process.

Transcription complexes that program Xenopus 5S RNA genes are stable in vivo.

The long-term stability of transcription complexes on 5S RNA genes has been demonstrated in vivo. Complexes on oocyte and somatic-type 5S RNA genes injected into Xenopus laevis oocyte nuclei are stable for at least 4 days. Tissue culture cells and mature erythrocytes have equivalent numbers of somatic 5S RNA genes programmed into transcription complexes, yet the former cell type has a greater than 50-fold higher cellular content of transcription factor IIIA (TFIIIA). Functional transcription complexes on somatic 5S RNA genes in nucleated erythrocytes of Xenopus are stable for weeks, perhaps months, even though a mature erythrocyte has less than two molecules of TFIIIA for each somatic 5S RNA gene. These findings strengthen our proposal that stable transcription complexes are a means of maintaining the differentiated state.

Transcriptionally inactive oocyte-type 5S RNA genes of Xenopus laevis are complexed with TFIIIA in vitro.

An extract from whole oocytes of Xenopus laevis was shown to transcribe somatic-type 5S RNA genes approximately 100-fold more efficiently than oocyte-type 5S RNA genes. This preference was at least 10-fold greater than the preference seen upon microinjection of 5S RNA genes into oocyte nuclei or upon in vitro transcription in an oocyte nuclear extract. The approximately 100-fold transcriptional bias in favor of the somatic-type 5S RNA genes observed in vitro in the whole oocyte extract was similar to the transcriptional bias observed in developing Xenopus embryos. We also showed that in the whole oocyte extract, a promoter-binding protein required for 5S RNA gene transcription, TFIIIA, was bound both to the actively transcribed somatic-type 5S RNA gene and to the largely inactive oocyte-type 5S RNA genes. These findings suggest that the mechanism for the differential expression of 5S RNA genes during Xenopus development does not involve differential binding of TFIIIA to 5S RNA genes.

Transcriptionally inactive oocyte-type 5S RNA genes of Xenopus laevis are complexed with TFIIIA in vitro.

An extract from whole oocytes of Xenopus laevis was shown to transcribe somatic-type 5S RNA genes approximately 100-fold more efficiently than oocyte-type 5S RNA genes. This preference was at least 10-fold greater than the preference seen upon microinjection of 5S RNA genes into oocyte nuclei or upon in vitro transcription in an oocyte nuclear extract. The approximately 100-fold transcriptional bias in favor of the somatic-type 5S RNA genes observed in vitro in the whole oocyte extract was similar to the transcriptional bias observed in developing Xenopus embryos. We also showed that in the whole oocyte extract, a promoter-binding protein required for 5S RNA gene transcription, TFIIIA, was bound both to the actively transcribed somatic-type 5S RNA gene and to the largely inactive oocyte-type 5S RNA genes. These findings suggest that the mechanism for the differential expression of 5S RNA genes during Xenopus development does not involve differential binding of TFIIIA to 5S RNA genes.

DNA sequence-directed nucleosome reconstitution on 5S RNA genes of Xenopus laevis.

Nucleosomes were reconstituted in vitro with several singly end-labeled restriction fragments derived from a cloned somatic-type 5S RNA gene of Xenopus laevis and purified nucleosome core particles from Xenopus cultured cells or chicken erythrocytes. Nucleosome locations were determined by digestion of the reconstitutes with exonuclease III and DNase I and were the same for all fragments investigated, extending from 20 base pairs (bp) within the 5S gene to 80 bp beyond the 3' end of the gene. Both core particles and crude nuclear extracts gave equivalent results, suggesting that no factors other than the core histones are responsible for recognition of DNA sequence during reconstitution. The histone octamer and the 5S gene-specific transcription factor TFIIIA both bind to the same region and face of 5S DNA, and nucleosome reconstitution on the 5S gene excluded binding of TFIIIA. The helical repeat of somatic-type 5S DNA in solution was measured by the band shift method and was 10.5 to 10.6 bp per turn over the region of the TFIIIA-binding site. The difference in helical repeat between DNA in solution and on the surface of the nucleosome (10.0-bp spacing between DNase I cutting sites) may explain the linking number paradox.

DNA sequence-directed nucleosome reconstitution on 5S RNA genes of Xenopus laevis.

Nucleosomes were reconstituted in vitro with several singly end-labeled restriction fragments derived from a cloned somatic-type 5S RNA gene of Xenopus laevis and purified nucleosome core particles from Xenopus cultured cells or chicken erythrocytes. Nucleosome locations were determined by digestion of the reconstitutes with exonuclease III and DNase I and were the same for all fragments investigated, extending from 20 base pairs (bp) within the 5S gene to 80 bp beyond the 3' end of the gene. Both core particles and crude nuclear extracts gave equivalent results, suggesting that no factors other than the core histones are responsible for recognition of DNA sequence during reconstitution. The histone octamer and the 5S gene-specific transcription factor TFIIIA both bind to the same region and face of 5S DNA, and nucleosome reconstitution on the 5S gene excluded binding of TFIIIA. The helical repeat of somatic-type 5S DNA in solution was measured by the band shift method and was 10.5 to 10.6 bp per turn over the region of the TFIIIA-binding site. The difference in helical repeat between DNA in solution and on the surface of the nucleosome (10.0-bp spacing between DNase I cutting sites) may explain the linking number paradox.

Differential order of replication of Xenopus laevis 5S RNA genes.

In Xenopus laevis there are two multigene families of 5S RNA genes: the oocyte-type 5S RNA genes which are expressed only in oocytes and the somatic-type 5S RNA genes which are expressed throughout development. The Xenopus 5S RNA replication-expression model of Gottesfeld and Bloomer (Cell 28:781-791, 1982) and Wormington et al. (Cold Spring Harbor Symp. Quant. Biol. 47:879-884, 1983) predicts that the somatic-type 5S RNA genes replicate earlier in the cell cycle than do the oocyte-type genes. Hence, the somatic-type 5S RNA genes have a competitive advantage in binding the transcription factor TFIIIA in somatic cells and are thereby expressed to the exclusion of the oocyte-type genes. To test the replication-expression model, we determined the order of replication of the oocyte- and somatic-type 5S RNA genes. Xenopus cells were labeled with bromodeoxyuridine, stained for DNA content, and then sorted into fractions of S phase by using a fluorescence-activated cell sorter. The newly replicated DNA containing bromodeoxyuridine was separated from the lighter, unreplicated DNA by equilibrium centrifugation and was hybridized with DNA probes specific for the oocyte- and somatic-type 5S RNA genes. In this way we found that the somatic-type 5S RNA genes replicate early in S phase, whereas the oocyte-type 5S RNA genes replicate late in S phase, demonstrating a key aspect of the replication-expression model.

Early replication and expression of oocyte-type 5S RNA genes in a Xenopus somatic cell line carrying a translocation.

In Xenopus somatic cells, the somatic-type 5S RNA genes replicate early in S phase, bind the transcription factor TFIIIA, and are expressed; in contrast, the late replicating oocyte-type genes do not bind TFIIIA and are transcriptionally inactive. These facts support a model in which the order of replication of the somatic-type versus the oocyte-type 5S genes causes their differential expression in somatic cells due to sequestration of TFIIIA by the early-replicating somatic genes. Here we provide further evidence for the model by showing that in one Xenopus cell line in which some oocyte-type 5S genes are translocated, some oocyte-type 5S genes replicate early and are expressed.

Differential Order of Replication of Xenopus laevis 5S RNA Genes

In Xenopus laevis there are two multigene families of 5S RNA genes: the oocyte-type 5S RNA genes which are expressed only in oocytes and the somatic-type 5S RNA genes which are expressed throughout development. The Xenopus 5S RNA replication-expression model of Gottesfeld and Bloomer (Cell 28:781-791, 1982) and Wormington et al. (Cold Spring Harbor Symp. Quant. Biol. 47:879-884, 1983) predicts that the somatic-type 5S RNA genes replicate earlier in the cell cycle than do the oocyte-type genes. Hence, the somatic-type 5S RNA genes have a competitive advantage in binding the transcription factor TFIIIA in somatic cells and are thereby expressed to the exclusion of the oocyte-type genes. To test the replication-expression model, we determined the order of replication of the oocyte- and somatic-type 5S RNA genes. Xenopus cells were labeled with bromodeoxyuridine, stained for DNA content, and then sorted into fractions of S phase by using a fluorescence-activated cell sorter. The newly replicated DNA containing bromodeoxyuridine was separated from the lighter, unreplicated DNA by equilibrium centrifugation and was hybridized with DNA probes specific for the oocyte- and somatic-type 5S RNA genes. In this way we found that the somatic-type 5S RNA genes replicate early in S phase, whereas the oocyte-type 5S RNA genes replicate late in S phase, demonstrating a key aspect of the replication-expression model.

Torsional stress induces an S1 nuclease-hypersensitive site within the promoter of the Xenopus laevis oocyte-type 5S RNA gene.

The internal promoter of the Xenopus laevis oocyte-type 5S RNA gene is preferentially cleaved by S1 and Bal-31 nucleases in plasmid DNA. S1 nuclease sensitivity is largely dependent on supercoiling; however, Bal-31 cleaves within the 5S RNA gene in linear as well as in supercoiled DNA. The S1 nuclease-hypersensitive site is centered at position +48-52 of the gene at the 5' boundary of the promoter. A DNAase I-hypersensitive site is induced at this position upon binding of the transcription factor, TFIIIA, specific for the 5S RNA gene. The somatic-type 5S RNA gene promoter is not preferentially cleaved by S1 nuclease or Bal-31 nuclease in supercoiled DNA, nor does TFIIIA induce a DNase I site at position +50. This differential promoter response may be related to a 4-fold difference in TFIIIA affinity between the oocyte and somatic 5S RNA genes.

Oocyte extracts reactivate developmentally inert Xenopus 5S genes in somatic nuclei.

In Xenopus and other amphibians, there are two major classes of 5S RNA gene1. One class, which codes for somatic-type 5S RNA, is expressed in both oocytes and somatic cells2; the other, which is 50 times more abundant3 and which codes for oocyte-type 5S RNA, is expressed in oocytes but not in somatic cells2,4. The developmentally inactivated oocyte-type genes are reactivated when erythrocyte nuclei are injected into most oocytes (activating oocytes)5. As the injected nuclei are induced to take on the natural activity of the host cell, the activation of oocyte-type 5S genes may represent a normal control mechanism. A special opportunity to investigate this mechanism is provided by the existence of certain individual animals whose oocytes consistently fail to activate 5S genes (non-activating oocytes). We have injected extracts of activating or non-activating oocytes, together with erythrocyte nuclei, into oocytes of each kind. We show here that the transcription of 5S genes is induced by extracts of activating oocytes (positive regulation) and is not inhibited by similar extracts of non-activating oocytes. The activating factor is sensitive to protease and heat treatment, suggesting that a protein is involved.

Developmental inactivity of 5S RNA genes persists when chromosomes are cut between genes.

Xenopus and other amphibians possess two major classes of 5S RNA genes1. One class, which codes for somatic-type 5S RNA, is expressed in both oocytes and somatic cells2. The other class, which is 50 times more abundant and encodes oocyte-type 5S RNA, is expressed in oocytes but not in somatic cells2–4. The oocyte-type 5S genes are located in tandem arrays of hundreds of genes at the ends of most chromosomes5, and it has been postulated that the lack of expression of oocyte-type 5S RNA genes in somatic cells is due to some general characteristic of the chromosomal region which the genes occupy6, for example, a folding into heterochromatin or higher-order structures. An alternative possibility is that the inactivity of these genes is controlled individually, and is independent of adjacent genetic material. We have now distinguished between these two possibilities by testing the transcription of oocyte-type 5S RNA genes either in their normal arrangement on intact chromosomes or after each gene has been separated from its neighbours by micrococcal nuclease digestion. Transcription was assayed by injecting intact nuclei, containing the 5S RNA genes as chromatin, into Xenopus oocytes7. We find that the developmental inactivity of oocyte-type 5S RNA genes is retained after the chromatin is cleaved into fragments the size of individual gene repeats.