The H1A histone variant is an in vivo repressor of oocyte-type 5S gene transcription in Xenopus laevis embryos.

The cleavage-stage (CS) histones of the sea urchin are known to be maternally expressed in the egg, have been implicated in chromatin remodeling of the male pronucleus following fertilization, and are the only histone variants present in embryonic chromatin up to the four-cell stage. With the help of partial peptide sequence information, we have isolated and identified CS H1, H2A, H2B, H3, and H4 cDNAs from egg poly(A) ؉ mRNA of the sea urchin Psammechinus miliaris. All five CS proteins correspond to replacement histone variants which are encoded by replication-independent genes containing introns, poly(A) addition signals, and long nontranslated sequences. Transcripts of the CS histone genes could be detected only during oogenesis and in development up to the early blastula stage. The CS proteins, with the exception of H4, are unique histones which are distantly related in sequence to the early, late, and sperm histone subtypes of the sea urchin. In contrast, the CS H1 protein displays highest sequence homology with the H1M (B4) histone of Xenopus laevis. Both H1 proteins are replacement histone variants with very similar developmental expression profiles in their respective species, thus indicating that the frog H1M (B4) gene is a vertebrate homolog of the CS H1 gene. These data furthermore suggest that the CS histones are of ancient evolutionary origin and may perform similar conserved functions during oogenesis and early development in different species.Histones are basic proteins that associate with each other and nuclear DNA to form the nucleosome. This basic unit of chromatin consists of two molecules each of the core histones H2A, H2B, H3, and H4, while the H1 protein interacts both with the histone octamer and with linker DNA and is responsible for packaging nucleosomes into higher-order structures. The core histones H3 and H4 are almost invariant in evolution. The less well conserved H2A and H2B proteins often occur as different subtypes within the same organism, while the H1 protein is the most variable of all histones (reviewed in reference 66).The expression of the majority of histone genes is tightly coupled to DNA synthesis by specific transcriptional and posttranscriptional mechanisms. These so-called replication histone genes code for mRNAs with short 5Ј and 3Ј untranslated regions; they lack introns and polyadenylation sequences but instead end in a 3Ј-terminal palindrome (11, 34) that serves as a recognition sequence for U7 snRNP-mediated 3Ј processing of histone pre-mRNA (5). A minor group of histone genes is expressed at a basal level throughout the cell cycle in proliferating cells and at a reduced but significant rate in quiescent cells, where their proteins gradually replace the replicationdependent histones in the chromatin (73, 74). These so-called replacement histone genes contain introns, code for polyadenylated mRNA with long untranslated sequences, and are thus classical RNA polymerase II transcription units (9,22,68).The sea urchin genome contains four distinct histone gene familie...

Section: Discussionmentioning

confidence: 99%

The Five Cleavage-Stage (CS) Histones of the Sea Urchin Are Encoded by a Maternally Expressed Family of Replacement Histone Genes: Functional Equivalence of the CS H1 and Frog H1M (B4) Proteins

Mandl

Brandt

Superti‐Furga

et al. 1997

“…Preferential binding of histone H1 has been described for the AT-rich DNA sequences of the SAR (17) as well as the AT-rich flanking sequences of the oocyte-type 5S RNA gene of Xenopus laevis (39), leading to the specific repression of the Xenopus oocyte 5S rRNA gene (6,16). In order to determine if histone H1 could also preferentially bind to the AT-rich DNA sequence present on the upstream region of the IFN-␤ promoter, we carried out a series of photofootprinting analyses of the muIFN-␤ promoter in the presence and absence of histone H1.…”

Section: High-affinity Binding Of the Hmgi Protein To The Upstream Atmentioning

confidence: 99%

Specific Binding of High-Mobility-Group I (HMGI) Protein and Histone H1 to the Upstream AT-Rich Region of the Murine Beta Interferon Promoter: HMGI Protein Acts as a Potential Antirepressor of the Promoter

Bonnefoy

Bandu

Doly

1999

The high-mobility-group I (HMGI) protein is a nonhistone component of active chromatin. In this work, we demonstrate that HMGI protein specifically binds to the AT-rich region of the murine beta interferon (IFN-␤) promoter localized upstream of the murine virus-responsive element (VRE). Contrary to what has been described for the human promoter, HMGI protein did not specifically bind to the VRE of the murine IFN-␤ promoter. Stably transfected promoters carrying mutations on this HMGI binding site displayed delayed virusinduced kinetics of transcription. When integrated into chromatin, the mutated promoter remained repressed and never reached normal transcriptional activity. Such a phenomenon was not observed with transiently transfected promoters upon which chromatin was only partially reconstituted. Using UV footprinting, we show that the upstream AT-rich sequences of the murine IFN-␤ promoter constitute a preferential binding region for histone H1. Transfection with a plasmid carrying scaffold attachment regions as well as incubation with distamycin led to the derepression of the IFN-␤ promoter stably integrated into chromatin. In vitro, HMGI protein was able to displace histone H1 from the upstream AT-rich region of the wild-type promoter but not from the promoter carrying mutations on the upstream high-affinity HMGI binding site. Our results suggest that the binding of histone H1 to the upstream AT-rich region of the promoter might be partly responsible for the constitutive repression of the promoter. The displacement by HMGI protein of histone H1 could help to convert the IFN-␤ promoter from a repressed to an active state.

“…This finding indicates that linker histones facilitate the in vitro folding of nucleosomal arrays but are not required for chromatin and chromosome assembly. In somatic cells of Xenopus, histone H1 was shown to function as a developmentally regulated gene repressor acting specifically on the set of embryonic 5S RNA genes (4,22,46) and mesoderm-specific genes (53). In the case of 5S RNA genes, molecular studies indicated that the repressive effect of histone H1 is related to differential nucleosome positioning (38,49).…”

mentioning

confidence: 99%

Histone H1 Is Dispensable for Methylation-Associated Gene Silencing in Ascobolus immersusand Essential for Long Life Span

Barra

Rhounim

Rossignol

et al. 2000

A gene encoding a protein that shows sequence similarity with the histone H1 family only was cloned in Ascobolus immersus. The deduced peptide sequence presents the characteristic three-domain structure of metazoan linker histones, with a central globular region, an N-terminal tail, and a long positively charged C-terminal tail. By constructing an artificial duplication of this gene, named H1, it was possible to methylate and silence it by the MIP (methylation induced premeiotically) process. This resulted in the complete loss of the Ascobolus H1 histone. Mutant strains lacking H1 displayed normal methylation-associated gene silencing, underwent MIP, and showed the same methylation-associated chromatin modifications as did wild-type strains. However, they displayed an increased accessibility of micrococcal nuclease to chromatin, whether DNA was methylated or not, and exhibited a hypermethylation of the methylated genome compartment. These features are taken to imply that Ascobolus H1 histone is a ubiquitous component of chromatin which plays no role in methylation-associated gene silencing. Mutant strains lacking histone H1 reproduced normally through sexual crosses and displayed normal early vegetative growth. However, between 6 and 13 days after germination, they abruptly and consistently stopped growing, indicating that Ascobolus H1 histone is necessary for long life span. This constitutes the first observation of a physiologically important phenotype associated with the loss of H1.