Modular structure of chromosomal proteins HMG-14 and HMG-17: definition of a transcriptional enhancement domain distinct from the nucleosomal binding domain.

Chromosomal proteins HMG-14 and HMG-17 are the only known nuclear proteins which specifically bind to the nucleosome core particle and are implicated in the generation and/or maintenance of structural features specific to active chromatin. The two proteins facilitate polymerase II and III transcription from in vitro- and in vivo-assembled circular chromatin templates. Here we used deletion mutants and specific peptides to identify the transcriptional enhancement domain and delineate the nucleosomal binding domain of the HMG-14 and -17 proteins. Deletion of the 22 C-terminal amino acids of HMG-17 or 26 C-terminal amino acids of HMG-14 reduces significantly the ability of the proteins to enhance transcription from chromatin templates. In contrast, N-terminal truncation mutants had the same transcriptional enhancement activity as the full-length proteins. We conclude that the negatively charged C-terminal region of the proteins is required for transcriptional enhancement. Chromatin transcription enhancement assays, which involve binding competition between the full-length proteins and peptides derived from their nucleosomal binding regions, indicate that the minimal nucleosomal binding domain of human HMG-17 is 24 amino acids long and spans residues 17 to 40. The results suggest that HMG-14 and -17 proteins have a modular structure and contain distinct functional domains.

Specific acetylation of chromosomal protein HMG-17 by PCAF alters its interaction with nucleosomes.

Nonhistone chromosomal proteins HMG-14 and HMG-17 are closely related nucleosomal binding proteins that unfold the higher-order chromatin structure, thereby enhancing the transcription and replication potential of chromatin. Here we report that PCAF, a transcription coactivator with intrinsic histone acetyltransferase activity, specifically acetylates HMG-17 but not HMG-14. Using mass spectrum sequence analysis, we identified the lysine at position 2 as the predominant site acetylated by PCAF. Lysine 2 is a prominent acetylation site in vivo, suggesting that this PCAF-mediated acetylation is physiologically relevant. Experiments with HMG-17 deletion mutants and competition studies with various protein fragments indicate that the specific acetylation of HMG-17 is not determined solely by the primary sequence near the acetylation site. By equilibrium dialysis we demonstrated that acetylation reduces the affinity of HMG-17 to nucleosome cores. In addition, we found that the binding of HMG-14 and HMG-17 to nucleosome cores inhibits the PCAF-mediated acetylation of histone H3. Thus, the presence of HMG-14 and HMG-17 affects the ability of PCAF to acetylate chromatin, while the acetylation of HMG-17 reduces its binding affinity to chromatin. Conceivably, in HMG-17-containing chromatin, acetylation of HMG-17 precedes the acetylation of histones.

Homodimers of chromosomal proteins HMG-14 and HMG-17 in nucleosome cores.

In this work, we report that nucleosome core particles interact with an equimolar mixture of the chromosomal proteins HMG-14 and HMG-17 to form, exclusively, complexes containing two molecules of either HMG-14 or HMG-17 (homodimers). Analysis of the binding of various mixtures of wild-type proteins and their deletion mutants indicates that homodimer formation is not dependent on contacts between the nucleosome-bound HMG-14/-17 proteins themselves. We suggest that HMG-14/-17 proteins in nucleosomes cross-talk by inducing specific allosteric transitions in the chromatin subunit.

Enhancement of the transcription potential of nascent chromatin by chromosomal proteins HMG-14/-17 is coupled to nucleosome assembly and not DNA synthesis.

We have previously demonstrated that in Xenopus egg extracts, which support DNA strand synthesis and chromatin assembly, incorporation of chromosomal proteins HMG-14/-17 into nascent nucleosomes increases the transcriptional potential of a chromatin template carrying the Xenopus 5S RNA gene. Here we use the single-stranded and double-stranded forms of a plasmid carrying a 5S RNA maxigene, to test whether the effect of HMG-14/-17 on transcription requires DNA synthesis and whether these proteins will affect transcription through a region containing nucleosomes. We find that most of the transcripts were about 350 nucleotides long, suggesting that HMG-14/-17 enhance transcription through a region that could contain nucleosomes. HMG-14/-17 enhance transcription of chromatin templates assembled onto double-stranded DNA, in the absence of DNA synthesis. Single-round transcription assays suggest that HMG-14/-17 increase transcription from templates assembled onto both single- and double-stranded DNA by increasing the specific activity, and not the number, of transcriptionally active templates. We conclude that the effect of HMG-14/-17 on the transcriptional potential of chromatin is linked to nucleosome assembly and is not linked to DNA synthesis.

Histone H1 in two subspecies of Chironomus thummi with different genome sizes: homologous chromosome sites differ largely in their content of a specific H1 variant.

Chromatin of Chironomus thummi (Diptera) contains seven sequence variants of histone H1. A structurally divergent H1, variant I-1, accounts for about 20% of the total H1 in C. th. piger and for about 30% in C. th. thummi. Monoclonal antibodies against this protein have been induced and have revealed its restriction to the centromeres and to a limited number of other bands in the salivary gland chromosomes. Indirect immunofluorescence of the somatically paired homologous chromosomes of F1 hybrids indicates that the difference between the two subspecies in H1 I-1 content largely depends on differences situated at a number of distinct homologous chromosome bands. These bands were intensely decorated by antibodies against H1 I-1 in C. th. thummi but appeared virtually black in C. th. piger. The same bands, however, were decorated equally in both subspecies by an antibody that reacts with other H1 variants but does not recognize H1 I-1 and by a polyclonal anti-H1 antibody. The results suggest that H1 variant I-1 is characteristic of a specific type of chromatin that is confined to distinct chromosome segments and that is more frequent in the subspecies C. th. thummi, which has a 27% larger genome.

The chromatin unfolding domain of chromosomal protein HMG-14 targets the N-terminal tail of histone H3 in nucleosomes.

Nonhistone chromosomal protein HMG-14 is a nucleosomal binding protein that unfolds the higher-order chromatin structure and enhances the transcriptional potential of chromatin, but not that of DNA. Both the transcriptional enhancement and the chromatin unfolding activities of HMG-14 are mediated through the C-terminal region of the protein. Here we study the molecular interactions of both this region and the N-terminal region of HMG-14 with nucleosome cores. By protein photocrosslinking we demonstrate that the N-terminal domain of HMG-14 targets a restricted region in histone H2B, whereas the C-terminal chromatin unfolding domain of HMG-14 targets a restricted region in the N terminus of histone H3. The N-terminal regions of the core histones are involved in the folding of oligonucleosomes and are the target of various activities associated with chromatin unfolding and transcriptional activation. We suggest that specific interactions between the C-terminal domain of HMG-14 and the N-terminal tail of histone H3 reduce the compaction of chromatin. These findings provide insights into the molecular mechanism whereby HMG-14/-17 proteins reduce the repressive effect of chromatin, and they also broaden the scope of the molecular interactions involving the N termini of the core histones in nucleosomes.

The HMG-14/-17 chromosomal protein family: architectural elements that enhance transcription from chromatin templates.

Chromosomal proteins HMG-14 and HMG-17 enhance the transcriptional potential of chromatin when incorporated into nucleosomes during, but not after, chromatin assembly on replicating DNA. Two molecules of either HMG-14 or HMG-17 can bind to nucleosome cores, independently of the underlying DNA sequence, in a cooperative fashion to limit nucleosome mobility and stabilize the structure of the nucleosome core without stabilizing the higher order chromatin structure. By modifying the structure of nucleosomes, the proteins affect the local structure of the chromatin fiber leading to an increase in the rate of transcriptional elongation but not initiation. We suggest that HMG-14/-17 are architectural elements which assist in the assembly of an unfolded chromatin fiber thereby decreasing the repressive activity of histones and facilitating transcriptional processes.

The histone H1 genes of the dipteran insect, Chironomus thummi, fall under two divergent classes and encode proteins with distinct intranuclear distribution and potentially different functions.

Four histone H1 genes of the midge, Chironomus thummi piger, and three H1 genes of the subspecies C. thummi thummi have been cloned and assigned to the four different H1 proteins from C. thummi larvae. Together with an earlier cloned H1 gene from C. thummi thummi [Hankeln, T. & Schmidt, E. R. (1991) Chromosoma 101, 25-31], these genes probably constitute the complete complement of H1 genes in both subspecies. They were found to fall under two classes that differ remarkably in their gene copy numbers, genomic organization, structure of flanking sequences, codon usage, and expression during embryonic development, and that encode H1 proteins of divergent structure. Histone H1 I-1 contains an inserted sequence, KAPKAPKAPKSPKAE in C. thummi piger, and KAPKAPKSPKAE in C. thummi thummi, that is lacking in the other H1 variants, H1 II-1, H1 II-2, and H1 III-1. In the immediate neighbourhood to the inserted sequence, a substitution in the H1 I-1 protein sequence dramatically enhances the potential to form a reversed turn. In early development, H1 I-1 is expressed at a higher rate than the other H1 genes. The transcripts have a size of about 1 kb; in addition, the H1 I-1 gene exhibited two minor transcripts of about 2.5 and > 3 kb size in middle blastoderm that are possibly polyadenylated. Together with our earlier finding that histone H1 I-1 is found in a limited number of polytene chromosome bands whereas the other H1 histones are uniformly distributed in chromatin, these results intimate functional differences between the two classes of H1 genes and their products.

Incorporation of chromosomal proteins HMG-14/HMG-17 into nascent nucleosomes induces an extended chromatin conformation and enhances the utilization of active transcription complexes.

The role of chromosomal proteins HMG-14 and HMG-17 in the generation of transcriptionally active chromatin was studied in a Xenopus laevis egg extract which supports complementary DNA strand synthesis and chromatin assembly. Chromosomal proteins HMG-14/HMG-17 enhanced transcription from a chromatin template carrying a 5S rRNA gene, but not from a DNA template. The transcriptional potential of chromatin was enhanced only when these proteins were incorporated into the template during, but not after, chromatin assembly. HMG-14 and HMG-17 stimulate transcription by increasing the activity, and not the number, of transcribed templates. They unfold the chromatin template without affecting the nucleosomal repeat or decreasing the content of histone B4. We suggest that HMG-14/HMG-17 enhance transcription by inducing an extended conformation in the chromatin fiber, perhaps due to interactions with histone tails in nucleosomes. By disrupting the higher order chromatin structure HMG-14/HMG-17 increase the accessibility of target sequences to components of the transcriptional apparatus.

Deposition of chromosomal protein HMG-17 during replication affects the nucleosomal ladder and transcriptional potential of nascent chromatin.

A cell-free system from Xenopus eggs was used to study the role of chromosomal protein HMG-17 in the generation of the chromatin structure of transcriptionally active genes. Addition of HMG-17 protein to the extracts, which do not contain structural homologs of the HMG-14/-17 protein family, indicates the protein is incorporated into the nascent template during replication, prior to completion of chromatin assembly. The protein binds to and stabilizes the structure of the nucleosomal core thereby improving the apparent periodicity of the nucleosomal spacing of nascent chromatin. Assembly of HMG-17 into the nascent chromatin structure significantly increased the transcription potential of the 5S RNA gene and satellite I chromatin. Kinetic studies indicate that the increase in transcriptional potential is observed only when HMG-17 is incorporated into nucleosomes during chromatin assembly.

Structural and functional differences between histone H1 sequence variants with differential intranuclear distribution.

The chromatin of most cell types contains several different sequence variants of histone H1. The functional role of this heterogeneity is not known. In the larval tissues of the midge, Chironomus thummi, there are H1 variants of two types. H1 II-1, H1 II-2, and H1 III-1 have similar amino acid sequences and appear uniformly distributed in polytene interphase chromosomes. The total number of gene copies per genome for this type of H1 histones is about 40 in C. th. thummi and 50-60 in C. th. piger. In contrast, histone H1 I-1 is encoded by a single copy gene in C. th. thummi and by two to four genes in C. th. piger. It has a divergent structure and is found only in a limited number of condensed chromosome sites. The N-terminal domain of H1 I-1 contains an insertion that is lacking in the other H1 variants and that is part of a variant-specific bipartite sequence Lys-Ala-Pro-Lys-Ala-Pro-Xaa10-Lys-Val-Ala in front of the conserved central domain. N-terminal peptides of H1 I-1 including this motif, in contrast to the homologous peptide from H1 II-1, competed with the drug Hoechst 33258 for binding to the minor groove of the DNA double helix. Repeats of the sequence Lys-Ala-Pro are also present at the same distance from the conserved central domain, in a single H1 variant of a nematode and of a green alga. The motif could interact with linker DNA in intranuclear targeting or packaging a condensed subtype of chromatin, or both.