32
2 Nucleic Acids and Nuclear Proteins
only 5-15 clusters spread throughout the
genome. The nucleotide sequences of the early
and late histone genes differ in the coding regions
by about 20 % and in the non-coding regions by
much more than this [291]. The large sequence
differences suggest that the separation of the two
types occurred a long time ago and is general for
the sea urchins.
2.5.3 Variability of the Histones
Non-allelic variants (subtypes) of histones are
now known in all histone classes except H4. Even
H3, despite its conservative character in the
mammals, is found as three somatic variants
(H3.1, H3.2 and H3.3) and a sperm-specific variant (TH3). The differentiation into H3.1 and
H3.2, where the only difference is Cys or Ser at
position 96, occurs only in mammals; the classes
H3.1 and H3.3, on the other hand, are found in
birds and other vertebrates. The H3.3 gene contains introns and probably represents far more the
original H3 gene; the intronless genes ofthe H3.1
class, however, possibly arose before the separation of plants and animals. Both H3.1 and H3.3
show no species-specific differences between
humans and chickens but differ from one another
at four positions. Histones of class H3.1 are only
synthesized in vertebrates during preparation for
nuclear division, but class 3.3 is synthesized
throughout the whole cell cycle. The H3.1 class
also resembles the early and late H3 of the sea
urchin [471]. Subtypes of the core histone H2A
are more than likely to be found in all vertebrates, but they also occur in the macronucleus of the
ciliate Tetrahymena, in Caenorhabditis elegans,
and in Drosophila melanogaster [98, 451].
Whereas the species-specific differences in histones H2A and H2B are usually localized in the Nterminal region, the variants found in the vertebrates also show differences in the C-terminal
region; this probably results in changes in histone
conformation and in structural and functional
properties of the whole nucleosome. The existence of different variants of core his tones thus
facilitates heterogeneity amongst the nucleosomes, and this may be important for the variable
expression of different DNA regions.
The linker histone Hl is the most variable of
all histones, and the various subtypes appear to
be correlated with metabolic activity and the
degree of condensation of the chromatin. Up to
seven Hl subtypes are found in somatic cells of
vertebrates and many invertebrates. Immunohistochemical investigations have shown that different chromatin regions in the giant chromosomes
of the midge Chironomus thummi have different
spectra of Hl subtypes [303]. The chicken is the
only animal in which a complete set of six Hl
sequences of 217-224 amino acids has been determined [84]. Two variants of the particularly
lysine-rich Hl° are to be found in all mammalian
organs; similar Hl variants are also present in all
other vertebrate classes and even in the mussel
Anodonta cygnea [300]. The proportion of the
Hl° type is not only cell-specific but also significantly higher in active regions of the genome: it
amounts to 12 % in the (inactive) globin region of
the liver cell genome of the mouse, 15-19 % in
the satellite DNA, and 61 % in the region of the
albumin gene [105]. Extremely aberrant histone
genes have been found in the duck Caraina moschata; the coding sequence of H3 differs from
that of the chicken in 10 of 135 amino acids, and
of Hl in 22 of 217-218 amino acids [439].
Unique histone variants are present in the
strongly condensed chromatin of nucleated erythrocytes and germline cells, and also in the transcriptionally inactive micronucleus of the Ciliophora. In bird erythrocytes, which retain their
nuclei after differentiation, an Hl variant, H5,
appears simultaneously with their genetic inactivation. H5 shows significant similarity to Hl° in
its central region and is also related to repression
of DNA synthesis. The proportion of erythrocytespecific histone increases after the end of DNA
synthesis [177]. Specific, as well as non-specific,
Hl variants may also be detected in the nucleated
erythrocytes of many fish, amphibians and reptiles, but do not always correspond electrophoretically with the H5 of bird erythrocytes. A typical H5
predominates in the teleosts Salmo gairdneri,
Perca flavescens and Pomoxis nigromaculatus;
Cyprinus carpio has little H5 but a lot of Hl; and
Catostomus commersoni has no H5. The erythrocytes of Xenopus laevis also contain both Hl and
H5; on the other hand, typical H5 was missing
from the red blood cells of Rana catesbeiana and
all reptiles that were examined [371].
The chromatin of mature male gametes is not
transcribed; it is highly condensed and thus the
genetic information is protected until the time of
fertilization. Sperm-specific histone variants are
responsible for the compact structure, as is protamine, the alternative type of nuclear protein.
During spermatogenesis in rats, there are dramatic changes in the spectrum of nuclear proteins: already in the spermatogonium there is a
large increase in the somatic forms Hla and
2 Nucleic Acids and Nuclear Proteins
only 5-15 clusters spread throughout the
genome. The nucleotide sequences of the early
and late histone genes differ in the coding regions
by about 20 % and in the non-coding regions by
much more than this [291]. The large sequence
differences suggest that the separation of the two
types occurred a long time ago and is general for
the sea urchins.
2.5.3 Variability of the Histones
Non-allelic variants (subtypes) of histones are
now known in all histone classes except H4. Even
H3, despite its conservative character in the
mammals, is found as three somatic variants
(H3.1, H3.2 and H3.3) and a sperm-specific variant (TH3). The differentiation into H3.1 and
H3.2, where the only difference is Cys or Ser at
position 96, occurs only in mammals; the classes
H3.1 and H3.3, on the other hand, are found in
birds and other vertebrates. The H3.3 gene contains introns and probably represents far more the
original H3 gene; the intronless genes ofthe H3.1
class, however, possibly arose before the separation of plants and animals. Both H3.1 and H3.3
show no species-specific differences between
humans and chickens but differ from one another
at four positions. Histones of class H3.1 are only
synthesized in vertebrates during preparation for
nuclear division, but class 3.3 is synthesized
throughout the whole cell cycle. The H3.1 class
also resembles the early and late H3 of the sea
urchin [471]. Subtypes of the core histone H2A
are more than likely to be found in all vertebrates, but they also occur in the macronucleus of the
ciliate Tetrahymena, in Caenorhabditis elegans,
and in Drosophila melanogaster [98, 451].
Whereas the species-specific differences in histones H2A and H2B are usually localized in the Nterminal region, the variants found in the vertebrates also show differences in the C-terminal
region; this probably results in changes in histone
conformation and in structural and functional
properties of the whole nucleosome. The existence of different variants of core his tones thus
facilitates heterogeneity amongst the nucleosomes, and this may be important for the variable
expression of different DNA regions.
The linker histone Hl is the most variable of
all histones, and the various subtypes appear to
be correlated with metabolic activity and the
degree of condensation of the chromatin. Up to
seven Hl subtypes are found in somatic cells of
vertebrates and many invertebrates. Immunohistochemical investigations have shown that different chromatin regions in the giant chromosomes
of the midge Chironomus thummi have different
spectra of Hl subtypes [303]. The chicken is the
only animal in which a complete set of six Hl
sequences of 217-224 amino acids has been determined [84]. Two variants of the particularly
lysine-rich Hl° are to be found in all mammalian
organs; similar Hl variants are also present in all
other vertebrate classes and even in the mussel
Anodonta cygnea [300]. The proportion of the
Hl° type is not only cell-specific but also significantly higher in active regions of the genome: it
amounts to 12 % in the (inactive) globin region of
the liver cell genome of the mouse, 15-19 % in
the satellite DNA, and 61 % in the region of the
albumin gene [105]. Extremely aberrant histone
genes have been found in the duck Caraina moschata; the coding sequence of H3 differs from
that of the chicken in 10 of 135 amino acids, and
of Hl in 22 of 217-218 amino acids [439].
Unique histone variants are present in the
strongly condensed chromatin of nucleated erythrocytes and germline cells, and also in the transcriptionally inactive micronucleus of the Ciliophora. In bird erythrocytes, which retain their
nuclei after differentiation, an Hl variant, H5,
appears simultaneously with their genetic inactivation. H5 shows significant similarity to Hl° in
its central region and is also related to repression
of DNA synthesis. The proportion of erythrocytespecific histone increases after the end of DNA
synthesis [177]. Specific, as well as non-specific,
Hl variants may also be detected in the nucleated
erythrocytes of many fish, amphibians and reptiles, but do not always correspond electrophoretically with the H5 of bird erythrocytes. A typical H5
predominates in the teleosts Salmo gairdneri,
Perca flavescens and Pomoxis nigromaculatus;
Cyprinus carpio has little H5 but a lot of Hl; and
Catostomus commersoni has no H5. The erythrocytes of Xenopus laevis also contain both Hl and
H5; on the other hand, typical H5 was missing
from the red blood cells of Rana catesbeiana and
all reptiles that were examined [371].
The chromatin of mature male gametes is not
transcribed; it is highly condensed and thus the
genetic information is protected until the time of
fertilization. Sperm-specific histone variants are
responsible for the compact structure, as is protamine, the alternative type of nuclear protein.
During spermatogenesis in rats, there are dramatic changes in the spectrum of nuclear proteins: already in the spermatogonium there is a
large increase in the somatic forms Hla and