the cytoplasm. The RNA of the cell nucleus consists mainly of the primary transcription products
of the three RNA polymerases, i.e. pre-rRNAs,
pre-tRNAs and the mRNA precursors; due to
their variable length these are known as heterogeneous nuclear RNAs (hnRNAs). These precursors and intermediates of RNA maturation show
a very high turnover rate. In addition, a group of
smaller, metabolically more stable RNA molecules, the snRNAs, is also found in the nucleus. All
the RNAs of the cell nucleus are organized, together with various proteins, into complex ribonucleoprotein (RNP) particles. The nuclear RNAs
are, for the most part, degraded in the nucleus;
only about 5 % of them reach the cytoplasm in
the form of ribosomes, tRNAs or mRNAs. The
cytoplasmic RNA consists mainly of rRNAs; the
tRNAs amount to about 10 % and the mRNAs
account for just a few percent. The latter are also
bound to various proteins in the cytoplasm to
form RNP particles, the informosomes. There
are, on the one hand, free informosomes with
non-translatable mRNA and, on the other hand,
the active mRNAs that are bound to ribosomes
(polysomes) and form loose associations with the
protein factors of translation and also with other
cytoplasmic proteins [276, 416]. In the oocytes of
amphibians and sea urchins, about 70 % of the
poly(A)-RNA is bound to transcripts of repetitive
DNA sequences; this combined RNA is not translated and disappears at the onset of embryo
development. This possibly represents a reserve
of immature mRNA [85].
2.8.1 The rRNA Genes
and Their 'fianscription
In all cells there exist organelles of about 20-nm
diameter, the ribosomes, on which protein biosynthesis takes place [178]. They are universally
constructed from two subunits of different size
that only combine with mRNA during protein
synthesis. In the ribosomes of E. coli, the bestknown ribosomes and the prototype for all the
rest, the smaller subunit contains an SS-rRNA
(small subunit) of 16S with 1540 nt and 21 different proteins, whereas the larger subunit has an
LS-rRNA (large subunit) of 23S (2900 nt), a 5S
rRNA (120 nt) and 32 proteins. The cytoplasmic
ribosomes of the eukaryotes are generally somewhat larger; the RNA of the small subunit has
18S (as many as 1800 nt), and the larger subunit
contains a 28S RNA (4800 nt), the 5S RNA, and
2.8.1 The rRNA Genes and Their Transcription
45
an additional5.8S rRNA (160 nt). The number of
proteins in both subunits is also larger than in
prokaryotes. The smallest eukaryotic ribosomes
are found in the Microsporidia, a group of parasitic unicellular organisms that exist in the cells of
many very different Eumetazoa and Protozoa;
these apparently separated quite early from the
evolutionary line of the eukaryotes. Their ribosomes are no larger than those of the prokaryotes
[460].
Mitochondria and plastids possess their own
systems for protein synthesis with a special type
of ribosome. The ribosomes of the mitochondria
are, without exception, smaller than prokaryotic
ribosomes, both subunits contain only one RNA
molecule each; the size of these mitochondrial
rRNAs varies from 12S + 16S in the higher eukaryotes to 9S + 12S in the trypanosomes [49, 326].
In spite of these extreme differences in size and
composition, the architecture of all ribosomes is
so similar that they can be traced back to a
common origin. Comparative investigations of
ribosome structure have led several authors to
classify the "Eocyta" as the fourth subkingdom of
living organisms, in addition to the Archaebacteria, Eubacteria and Eukaryotes. This is a group of
sulphur bacteria that live in hot springs and are
closer than the other two subkingdoms to the
eukaryotes in their ribosome structure [247].
According to other authors, however, ribosome
morphology alone does not provide satisfactory
evidence to justify such far-reaching conclusions
[335].
The coding sequences of the 18S rRNA, 5.8S
rRNA and 28S rRNA together make up the prerRNA genes (rDNA), which are organized in tandem repeats in the nucleolus. In all animals, in
contrast to the situation in yeast, the genes for the
5S rRNA are separated from the other rRNA
types. The number of pre-rRNA genes in the
Eumetazoa lies between 45 and several thousand
(Table 2.4), but the number of rRNA genes can
differ quite markedly between different individuals of a species, as has been seen, for example, in
Drosophila [156]. In the haploid genome of the
malarial agent Plasmodium berghei there are only
four rRNA genes, but these are, however, very
different in both length and sequence and are
expressed in a stage-specific manner [100]. The
ciliate Tetrahymena has only one rRNA gene per
haploid genome in the micronucleus, but in the
non-chromosomal DNA of the macronucleus
there are about 10 000 hairpin-like rDNA molecules that together amount to several percent of the
total DNA [374].
of the three RNA polymerases, i.e. pre-rRNAs,
pre-tRNAs and the mRNA precursors; due to
their variable length these are known as heterogeneous nuclear RNAs (hnRNAs). These precursors and intermediates of RNA maturation show
a very high turnover rate. In addition, a group of
smaller, metabolically more stable RNA molecules, the snRNAs, is also found in the nucleus. All
the RNAs of the cell nucleus are organized, together with various proteins, into complex ribonucleoprotein (RNP) particles. The nuclear RNAs
are, for the most part, degraded in the nucleus;
only about 5 % of them reach the cytoplasm in
the form of ribosomes, tRNAs or mRNAs. The
cytoplasmic RNA consists mainly of rRNAs; the
tRNAs amount to about 10 % and the mRNAs
account for just a few percent. The latter are also
bound to various proteins in the cytoplasm to
form RNP particles, the informosomes. There
are, on the one hand, free informosomes with
non-translatable mRNA and, on the other hand,
the active mRNAs that are bound to ribosomes
(polysomes) and form loose associations with the
protein factors of translation and also with other
cytoplasmic proteins [276, 416]. In the oocytes of
amphibians and sea urchins, about 70 % of the
poly(A)-RNA is bound to transcripts of repetitive
DNA sequences; this combined RNA is not translated and disappears at the onset of embryo
development. This possibly represents a reserve
of immature mRNA [85].
2.8.1 The rRNA Genes
and Their 'fianscription
In all cells there exist organelles of about 20-nm
diameter, the ribosomes, on which protein biosynthesis takes place [178]. They are universally
constructed from two subunits of different size
that only combine with mRNA during protein
synthesis. In the ribosomes of E. coli, the bestknown ribosomes and the prototype for all the
rest, the smaller subunit contains an SS-rRNA
(small subunit) of 16S with 1540 nt and 21 different proteins, whereas the larger subunit has an
LS-rRNA (large subunit) of 23S (2900 nt), a 5S
rRNA (120 nt) and 32 proteins. The cytoplasmic
ribosomes of the eukaryotes are generally somewhat larger; the RNA of the small subunit has
18S (as many as 1800 nt), and the larger subunit
contains a 28S RNA (4800 nt), the 5S RNA, and
2.8.1 The rRNA Genes and Their Transcription
45
an additional5.8S rRNA (160 nt). The number of
proteins in both subunits is also larger than in
prokaryotes. The smallest eukaryotic ribosomes
are found in the Microsporidia, a group of parasitic unicellular organisms that exist in the cells of
many very different Eumetazoa and Protozoa;
these apparently separated quite early from the
evolutionary line of the eukaryotes. Their ribosomes are no larger than those of the prokaryotes
[460].
Mitochondria and plastids possess their own
systems for protein synthesis with a special type
of ribosome. The ribosomes of the mitochondria
are, without exception, smaller than prokaryotic
ribosomes, both subunits contain only one RNA
molecule each; the size of these mitochondrial
rRNAs varies from 12S + 16S in the higher eukaryotes to 9S + 12S in the trypanosomes [49, 326].
In spite of these extreme differences in size and
composition, the architecture of all ribosomes is
so similar that they can be traced back to a
common origin. Comparative investigations of
ribosome structure have led several authors to
classify the "Eocyta" as the fourth subkingdom of
living organisms, in addition to the Archaebacteria, Eubacteria and Eukaryotes. This is a group of
sulphur bacteria that live in hot springs and are
closer than the other two subkingdoms to the
eukaryotes in their ribosome structure [247].
According to other authors, however, ribosome
morphology alone does not provide satisfactory
evidence to justify such far-reaching conclusions
[335].
The coding sequences of the 18S rRNA, 5.8S
rRNA and 28S rRNA together make up the prerRNA genes (rDNA), which are organized in tandem repeats in the nucleolus. In all animals, in
contrast to the situation in yeast, the genes for the
5S rRNA are separated from the other rRNA
types. The number of pre-rRNA genes in the
Eumetazoa lies between 45 and several thousand
(Table 2.4), but the number of rRNA genes can
differ quite markedly between different individuals of a species, as has been seen, for example, in
Drosophila [156]. In the haploid genome of the
malarial agent Plasmodium berghei there are only
four rRNA genes, but these are, however, very
different in both length and sequence and are
expressed in a stage-specific manner [100]. The
ciliate Tetrahymena has only one rRNA gene per
haploid genome in the micronucleus, but in the
non-chromosomal DNA of the macronucleus
there are about 10 000 hairpin-like rDNA molecules that together amount to several percent of the
total DNA [374].