Andrzej Kowalski

Linker histone subtypes and their allelic variants

Members of histone H1 family bind to nucleosomal and linker DNA to assist in stabilization of higher‐order chromatin structures. Moreover, histone H1 is involved in regulation of a variety of cellular processes by interactions with cytosolic and nuclear proteins. Histone H1, composed of a series of subtypes encoded by distinct genes, is usually differentially expressed in specialized cells and frequently non‐randomly distributed in different chromatin regions. Moreover, a role of specific histone H1 subtype might be also modulated by post‐translational modifications and/or presence of polymorphic isoforms. While the significance of covalently modified histone H1 subtypes has been partially recognized, much less is known about the importance of histone H1 polymorphic variants identified in various plant and animal species, and human cells as well. Recent progress in elucidating amino acid composition‐dependent functioning and interactions of the histone H1 with a variety of molecular partners indicates a potential role of histone H1 polymorphic variation in adopting specific protein conformations essential for chromatin function. The histone H1 allelic variants might affect chromatin in order to modulate gene expression underlying some physiological traits and, therefore could modify the course of diverse histone H1‐dependent biological processes. This review focuses on the histone H1 allelic variability, and biochemical and genetic aspects of linker histone allelic isoforms to emphasize their likely biological relevance.

Allelic Polymorphism of Histone H1.a in Duck Erythrocytes

In each eukaryotic cell ® ve classes of histones are found. Four of them (H2A, H2B, H3, and H4) play a key role in the organization of the nucleosome core. These proteins are evolutionarily conserved, in contrast to histone H1, which has been reported to vary in its structure within cells, tissues, individuals, and species (Køyszejko-Stefanowicz et al., 1989). Allelic polymorphism of histone H1 has been found in erythrocytes of some bird species [quail and goose (Paøyga, 1991c, 1990a)], rabbit liver (Paøyga, 1990b), and mouse spermatocytes (Zweidler, 1984). In the present work, polymorphic forms of histone H1.a, a nonallelic subtype of histone H1 of duck erythrocytes, have been identi® ed.

Sequence variants of chicken linker histone H1.a

Two allelic isoforms (H1.a1 and H1.a2) of histone H1.a were identified within two conservative flocks (R11 and R55) of Rhode Island Red chickens. These proteins form three phenotypes: a1, a2 and a1a2. Birds with phenotype a1 were most common (frequency 0.825–0.980) while the a1a2 chickens appeared relatively rarely (0.017–0.175). The third phenotype a2, not detected in the tested populations, has only been revealed in progeny of the purpose‐mated a1a2 birds. The polymorphism of histone H1.a was observed in all examined chicken tissues, so that the H1 preparations isolated from the lung, spleen, kidney and testis from the same individual exhibited identical phenotypes (a1, a2, or a1a2). This finding, together with inheritance data, supports the genetic nature of the H1.a polymorphism. As indicated by cleavages with α‐chymotrypsin and protease V8, the H1.a1 and H1.a2 are two highly related proteins which differ within N‐terminal part of their C‐terminal tails. Only a single nonconservative amino acid substitution between both H1.a allelic isoforms was detected by Edman degradation: glutamic acid present at position 117 in histone H1.a1 was replaced by lysine in histone H1.a2. Furthermore, using microsequencing techniques we have found a sequence homology between the N‐ and C‐terminal parts of an unknown minor protein H1.y, present in the phenotype a2, and similar regions of histone H1.b.

Chromatin compaction in terminally differentiated avian blood cells: the role of linker histone H5 and non-histone protein MENT

Chromatin has a tendency to shift from a relatively decondensed (active) to condensed (inactive) state during cell differentiation due to interactions of specific architectural and/or regulatory proteins with DNA. A promotion of chromatin folding in terminally differentiated avian blood cells requires the presence of either histone H5 in erythrocytes or non-histone protein, myeloid and erythroid nuclear termination stage-specific protein (MENT), in white blood cells (lymphocytes and granulocytes). These highly abundant proteins assist in folding of nucleosome arrays and self-association of chromatin fibers into compacted chromatin structures. Here, we briefly review structural aspects and molecular mode of action by which these unrelated proteins can spread condensed chromatin to form inactivated regions in the genome.

Modulation of chromatin function through linker histone H1 variants.

In this review, the structural aspects of linker H1 histones are presented as a background for characterization of the factors influencing their function in animal and human chromatin. The action of H1 histone variants is largely determined by dynamic alterations of their intrinsically disordered tail domains, posttranslational modifications and allelic diversification. The interdependent effects of these factors can establish dynamic histone H1 states that may affect the organization and function of chromatin regions.

Two polymorphic linker histone loci in Guinea fowl erythrocytes

A variable migration of linker histone H1.b and H1.c spots in two-dimensional polyacrylamide gel patterns of total erythrocyte histone H1 has been detected during population screening in two differently plumaged Guinea fowl strains. Alloforms, H1.b1 and H1.b2 as well as H1.c1 and H1.c2, differing in apparent molecular weights tended to form only phenotypes b1 and b2 or c1 and c2 in a white-feathered strain while all phenotypes (b1, b2 and b1b2 or c1, c2 and c1c2, respectively) were present in a black-feathered population. Accordingly, the white-feathered population significantly deviated from the Hardy-Weinberg principle (chi-square test, d.f=1, p<<0.001) due to a lack of heterozygotes while the black-feathered population conformed to the Hardy-Weinberg equilibrium (p>0.05) at both H1.b and H1.c loci. Differential electrophoretic mobilities of the C-peptides from a partial chemical cleavage (N-bromosuccinimide) or limited enzymatic digestion (α-chymotrypsin and protease V8) of the histone H1.b and H1.c alloforms seem to indicate that altered amino acid sequence segments might be located either at the C-terminal end of globular domain or in the C-terminal domain itself.

Natural allelic variation of duck erythrocyte histone H1b.

In our previous work (J. Palyga, Genetic polymorphisms of histone H1. b in duck erythrocytes. Hereditas 114, 85-89, 1991) we reported a genetic polymorphism of duck erythrocyte histone H1.b. Here, we screened H1 preparations in a two-dimensional polyacrylamide gel to refine the distribution of allelic forms of H1.b in fifteen duck populations. We have revealed that the frequency of H1.b allelic variants was significantly different among many conservative and breeding duck groups. While b(1) and b(3) were common in all populations screened, the allele b(2), with a slightly lower apparent molecular weight, was confined mainly to brown-feathered ducks (Khaki Campbell and Orpington) and descendent lines. The C- and N-terminal peptides released upon cleavage with N-bromosuccinimide and Staphylococcus aureus protease V8 from duck allelic histones H1. b2 and H1.b3, respectively, migrated differently in the gel, probably as a result of potential amino acid variation in a C-terminal domain.

Phenotypic variation of erythrocyte linker histone H1.c in a pheasant (Phasianus colchicus L.) population

Our goal was to characterize a phenotypic variation of the pheasant erythrocyte linker histone subtype H1.c. By using two-dimensional polyacrylamide gel electrophoresis three histone H1.c phenotypes were identified. The differently migrating allelic variants H1.c1 and H1.c2 formed either two homozygous phenotypes, c1 and c2, or a single heterozygous phenotype, c1c2. In the pheasant population screened, birds with phenotype c2 were the most common (frequency 0.761) while individuals with phenotype c1 were rare (frequency 0.043).

Genetic variants of chicken erythrocyte histone H5

Two allelic electromorphs a and b of chicken eryhrocyte histone H5 have been detected in a sodium dodecyl sulfate polyacrylamide gel. In an acid‐urea gel, however, each of the allelic variants was found to be accompanied by a slower migrating form. A comparison of α‐chymotrypsin‐digested products of H5.a and H5.b revealed that they differed in N‐terminal domains. The H5 variants were distributed differently not only in various chicken races but also in distinct lines within a breed. Allele H5b was about 2.6‐4.6 as abundant as its counterpart H5a in most chicken populations examined. These proportions were distorted in two Leghorn lines: the ratio of H5b to H5a was only 1.6 in line H22 and increased up to 32 in line G99.

Polymorphic linker histone H1 variants in breeding and conservative duck populations

Abstract A purpose of this study was to evaluate genetic diversity in duck populations based on polymorphic variants (H1.a, H1.b and H1.z) of linker histone H1. The study was performed using conservative brown-feathered Khaki Campbell (Kh1) and Orpington (Or) populations and white-feathered Pekin (P77) duck breeding line. While no significant distortion between both brown-feathered duck populations was noted (P>0.05), the allele frequencies at histone H1 polymorphic loci were found to differ significantly between brown-feathered and white-feathered duck flocks (P<0.001). While the alleles a1, b1 and z1 were detected in all three duck lines, the alleles a2 and b2 missed in the line P77 were found in the Kh1 and Or populations. A rare allele z2 not detected in a homozygous state during screening our duck populations was found to occur only in heterozygous P77 duck individuals (z1z2). After a purpose mating of these heterozygous P77 birds, the homozygous individuals (z2) were obtained in their offspring. The uneven distribution of the alleles for polymorphic histone H1 variants among duck populations seems to suggest that they are not functionally equivalent and, therefore, might have a differential influence on chromatin structure and/or function leading to line-specific phenotypic effects in duck.