Monocistronic transcription is the physiological mechanism of sea urchin embryonic histone gene expression

Psoralen crosslinking of active and inactive sea urchin histone and rRNA genes

Chromatin structure is highly correlated with the transcriptional activity of specific genes. For example, it has been found that the regularity of nucleosome spacing is compromised when genes are transcribed. The rRNA genes from fungi, plants, and animals give distinctly bimodal distributions of psoralen crosslinking, which has led to the suggestion that these genes might be largely devoid of nucleosomes when transcriptionally active. We investigated the chromatin structure of the multicopy rRNA and histone genes during sea urchin early embryogenesis. The rRNA genes, which are weakly expressed, give a unimodal distribution of weak psoralen crosslinking, in contrast to the situation in all other organisms studied. The early histone genes were more accessible to psoralen crosslinking when active than inactive. The pattern of crosslinking suggests that these polII genes have a homogeneous structure and are still highly protected by nucleosomes when in the active conformation, unlike the situation in polI genes.

The role of cap methylation in the translational activation of stored maternal histone mRNA in sea urchin embryos

Cap methylation was examined in the early sea urchin embryo. Nucleotide analyses of 3H-methyl methionine-labeled RNA in two-cell embryos and in unfertilized eggs show that fertilization activates the cap methylation of about 10(7) RNA molecules. Greater than 37% of methyl-labeled RNAs following fertilization hybridize with so-called early histone genes H1, H4, and H2B, which encode a subpopulation of the maternal mRNA molecules. Activation of RNA cap methylation is inhibited by aphidicolin, but not by actinomycin D, suggesting that this process is temporally coordinated with DNA replication, but independent of RNA transcription. These results indicate that the translational activation of maternal early histone mRNA during fertilization is a consequence of cap methylation of mRNAs incompletely formed during oogenesis.

The role of RNA polymerase initiation and elongation in control of total RNA and histone mRNA synthesis in sea urchin embryos

The involvement of RNA polymerase initiations in regulating total RNA synthesis and the synthesis of the early histone mRNAs was investigated. Nuclei were isolated from developing sea urchin embryos from 4- to 600-cell stages, and the transcription of already initiated polymerase complexes was studied in a "run-off," or elongation, assay; this assay was optimized by using high levels of ribonucleoside triphosphates. Under these conditions the relative levels of RNA synthesis in isolated nuclei from different stages closely paralleled the known rates of synthesis in vivo. However, if sarkosyl is included in the elongation assay, the nuclei of older stages display greatly stimulated synthesis while early cleavage stage nuclei are not stimulated. Sarkosyl does not reveal any elongated transcripts from the early histone genes in nuclei from later stages of development. This has been interpreted to mean that there are many initiated polymerase II complexes that do not elongate rapidly at later stages, but the early histone genes are inactive at later stages because they do not possess any productively initiated polymerases.

Evolving sea urchin histone genes--nucleotide polymorphisms in the H4 gene and spacers of Strongylocentrotus purpuratus

We present a comparison of spacer and coding sequences of histone gene repeats from four Strongylocentrotus purpuratus individuals. Sequences of two previously cloned units (pCO2 and pSp2) were compared with three new histone gene clones, two of them from a single individual. Within a 1.7-kb region, 59 polymorphic sites were found in spacers, in mRNA nontranslated stretches, and at silent sites in codons of the H4 gene. The permitted silent-site changes were as frequent as in any other region studied. The most abundant polymorphisms were single-base substitutions. The ratio of transitions : transversions : single-base-pair insertions/deletions was 3:2:2. A number of larger insertions/deletions were found, as well as differences in the length of (CTA)n and (CT)n runs. Two of the five cloned repeats contained an insertion of a 195-bp element that is also present at many other sites in the genomes of every S. purpuratus individual studied. Pairwise comparisons of the different clones indicate that the variation is not uniformly divergent, but ranges from a difference of 0.34% to 3.0% of all nucleotide sites. A parsimonious tree of ancestry constructed from the pairwise comparisons indicates that recombination between the most distantly related repeats has not occurred in the 1-2 million years necessary for accumulation of the variation. The level of sequence variation found within the S. purpuratus population, for both tandemly repeated and single-copy genes, is 25%-50% of that found between S. purpuratus and S. drobachiensis.

Most early-variant histone mRNA is contained in the pronucleus of sea urchin eggs

Previous studies demonstrated that the pronucleus of the unfertilized sea urchin egg contains a high concentration of transcripts complementary to the early histone repeat unit (D. L. Venezky, L. M. Angerer, and R. C. Angerer (1981). Cell 24, 385-391.) In this paper, in situ hybridization techniques of improved sensitivity are used to show that these nuclear RNAs include authentic histone mRNA but not spacer-complementary sequences. It is estimated that most early-variant histone mRNA contained in the egg is, in fact, restricted to the pronucleus. These mRNAs are released to the cytoplasm at the time of nuclear breakdown of first cleavage and rapidly distribute throughout the cytoplasm.

Timing and rates of synthesis of early histone mRNA in the embryo of Strongylocentrotus purpuratus

The level of early histone mRNA in Strongylocentrotus purpuratus changes abruptly at 6 hr of development, increasing an average of 10-fold by 9-10.5 hr and then decreasing over 2-fold by 13.5-15 hr. These changes occur when the late embryonic mRNA is still a very minor component of histone gene transcription. The exact values of increase and decrease of mRNA level vary from experiment to experiment and may reflect the conditions of embryos at different times. The instantaneous rate of synthesis of histone RNA per embryo increases from at least 47 fg/min at 6 hr to 114 fg/min at 9 hr and then drops to 29 fg/min at 12 hr. The rate of mRNA accumulation is lower: 20, 43, and 12 fg/min, respectively. On a per cell basis, however, the rate of synthesis and accumulation is highest at 6 hr and continuously decreases to 1/20 the level per cell at 12 hr. The transcriptional rates and relative mRNA increases taken together predict an average increase from 0.16 to 0.24 pg/embryo (6-10 X 10(5) molecules) per mRNA species in the egg to 1.6 to 2.4 pg/embryo (6-10 X 10(6) molecules) at 10.5 hr. The transcription rates indicate that at the maximal values we obtained, about two to three molecules of each histone RNA are made per gene copy per minute. The half-life of the histone mRNAs in the period from 6 to 13.5 hr probably varies, with the maximal turnover at about the time histone RNA level peaks. A half-life of 1.5 hr at 12 hr of development is estimated. Change in transcriptional rate per nucleus, increase in cell number, and probably a change in mRNA stability as well are therefore involved in the control of histone mRNA levels in the early embryo.

Individual regulation of the accumulation of H1 mRNA and core histone mRNAs in sea urchin embryos

The relative cytoplasmic accumulation of the individual histone mRNAs in sea urchins was determined by gel analysis of 3H-labeled cytoplasmic RNA isolated from embryos of the early cleavage through the mesenchyme blastula stages. A number of separate determinations showed that H1 mRNA accumulates at a molar ratio of 0.5 or less compared with each of the H2 or H3 core histone mRNAs through approximately the first 12 h of embryonic development. After this time, the accumulation of H1 mRNA increases relative to the core histone mRNAs, and approximately equimolar amounts of the histone mRNAs are produced by about the 14-h stage. The equimolar synthesis of H1 mRNA appears to be transient, returning to 0.5-molar levels several hours later. The increase in H1 mRNA accumulation, relative to the core histone RNAs, is coincident with the transition from expression of the early (alpha) sea urchin histone gene set to the late histone genes. Since all five of the early histone genes occur in a 1:1 ratio within repeating units, the data suggest that the genes within a single repeat, or their immediate products, are individually regulated. Gel analysis of the proteins synthesized in vivo by embryos demonstrates that the pattern of synthesis of the histone proteins reflects the changing ratios of the histone mRNAs.

Generation of authentic 3' termini of an H2A mRNA in vivo is dependent on a short inverted DNA repeat and on spacer sequences

We have determined what sequences are required to generate the authentic 3' termini of a sea urchin H2A histone mRNA. We have constructed a series of deletion and insertion mutants in the cloned histone repeat unit h22 of Psammechinus miliaris and have analyzed the transcripts of both wild-type and mutant DNAs produced in the frog oocyte. The protein-coding sequences of the H2A gene can be removed without any deleterious effects on transcription initiation or termination. A 12 bp deletion, which removes a highly conserved inverted DNA repeat immediately preceding the H2A mRNA 3' terminus, elicits read-through of the polymerase into the spacer DNA further downstream. However, the inverted repeat and the sequence coding for the 3' terminus of the mRNA are by themselves not sufficient to generate faithful 3' ends. Our data suggest that spacer sequences downstream of the 3' mRNA terminus are required as well.