28S
Fig. 2.15. The structure of the spacers between the rRNA
repeats of Xenopus laevis. The rRNA possesses two promoters (white); the non-transcribed spacer (NTS) contains
Amongst the vertebrates, only the salamander
Triturus vulgaris has an intron, and this occurs in
8 % of the rDNA repeats; they are not found in
other species of Triturus or in other amphibian
genera like Xenopus [270]. In view of the extensive similarities in the structural and functional
characters of the rRNAs, the variation in gene
organization, with differences in location, number of genes, lengths of spacers, presence or
absence of introns etc., can hardly be looked
upon as adaptive evolution.
About one-half of the total transcriptional
activity of the cell is devoted to the transcription
of the rDNA, and this forms the single function
of RNA polymerase I. One mammalian cell
requires in the order of 2 million new ribosomes
before each cell division; with a generation time
of 15 h, a Pol I transcription rate of 30 ntis and a
distance of 100 bp between the Pol I molecules, a
minimum of 50 rRNA genes per haploid genome
is required to cover the rRNA needs of the cell,
and, in fact, the number of rRNA repeats is
mostly higher (Table 2.4). A closer study of
rDNA transcription has only become possible
within the last few years since in vitro and in vivo
systems have become available for transcribing
cloned rDNA. The primary transcript (prerRNA) includes significant parts of the spacers
(ETS) from in front of and behind the coding
sequence, as well as the coding sequence itself
and the internal spacers (ITS). At 47S, this prerRNA in mammals is larger than was originally
thought (45S) , but the 5'-terminal 650 nt are
quickly removed; on the other hand, in Xenopus
and Drosophila the 5'-terminus of the pre-rRNA
is identical to that of the 18S rRNA. Contrary to
previous ideas, transcription, at least in mammals
and Xenopus, proceeds far beyond the end of the
28S rRNA [411].
The Pol I promoters, in contrast to those of
Pol II and Pol III, appear to be taxon specific.
Thus, there is no transcription of mouse rRNA in
extracts of human cells, or of Drosophila virilis
rDNA in extracts of D. melanogaster; however,
NTS
2.8.2 Structure of rRNAs
47
18 S
r--promoters~
(rIl •••••••• ) 1'1
60 / 81 bp -
n = 2_ 6 L--- - -- - - - l
Repeats
- - -- l k b -
two to six segments of 1 kb, made up of repeated elements
of 60 and 81 bp that function as enhancers (black). These
segments contain further promoters [411]
Xenopus laevis and X. borealis are compatible, as
are the mouse and rat, and also man and several
ape species. The reason for this appears to lie
with proteinaceous transcription factors that
show species-specific sequence recognition. However, positive results have been reported with the
rDNA of the beetle Dytiscus in oocytes of the
clawed frog Xenopus, and with Xenopus rDNA in
mouse cells. The efficiency of Pol I-catalysed
transcription is apparently strongly dependent
upon the experimental conditions. Thus, for initiation of transcription in Xenopus the "proximal
promoter", the region between -40 and + 10
(where the initiation site is + 1), is in many cases
sufficient, whereas in other cases optimal transcription requires additional upstream promoters
between -150 and -110. In Xenopus there are further promoters lying 1 or more kb in front of the
initiation site; in addition, there are repetitive
sequences of 60 or 81 bp that act as enhancers to
increase transcription (Fig. 2.15) [411]. The trypanosomes, whose Pol II transcripts with the later
added "spliced leader" sequence are so unusual
(p. 39), apparently have quite normal rDNA transcripts [118].
2.8.2 Structure of rRNAs
All known sequences of SS-rRNAs, LS-rRNAs
and 5.8S rRNAs, together with those of the 5S
rRNAs, tRNAs and snRNAs, are published
regularly in supplementary issues of the journal
Nucleic Acids Research. The length of the SSrRNA in the small ribosomal subunit varies
between 9S (about 640 nt) and 18S (about
1870 nt), and that of the LS-rRNA in the large
subunit varies between 12S (about 1230 nt) and
28S (about 4800 nt). Nevertheless, the primary
and secondary structures of both rRNA classes
are very similar. The differences in length are
the result of insertions in otherwise homologous
regions which do not significantly disturb the
secondary structure of the central region. Open
Fig. 2.15. The structure of the spacers between the rRNA
repeats of Xenopus laevis. The rRNA possesses two promoters (white); the non-transcribed spacer (NTS) contains
Amongst the vertebrates, only the salamander
Triturus vulgaris has an intron, and this occurs in
8 % of the rDNA repeats; they are not found in
other species of Triturus or in other amphibian
genera like Xenopus [270]. In view of the extensive similarities in the structural and functional
characters of the rRNAs, the variation in gene
organization, with differences in location, number of genes, lengths of spacers, presence or
absence of introns etc., can hardly be looked
upon as adaptive evolution.
About one-half of the total transcriptional
activity of the cell is devoted to the transcription
of the rDNA, and this forms the single function
of RNA polymerase I. One mammalian cell
requires in the order of 2 million new ribosomes
before each cell division; with a generation time
of 15 h, a Pol I transcription rate of 30 ntis and a
distance of 100 bp between the Pol I molecules, a
minimum of 50 rRNA genes per haploid genome
is required to cover the rRNA needs of the cell,
and, in fact, the number of rRNA repeats is
mostly higher (Table 2.4). A closer study of
rDNA transcription has only become possible
within the last few years since in vitro and in vivo
systems have become available for transcribing
cloned rDNA. The primary transcript (prerRNA) includes significant parts of the spacers
(ETS) from in front of and behind the coding
sequence, as well as the coding sequence itself
and the internal spacers (ITS). At 47S, this prerRNA in mammals is larger than was originally
thought (45S) , but the 5'-terminal 650 nt are
quickly removed; on the other hand, in Xenopus
and Drosophila the 5'-terminus of the pre-rRNA
is identical to that of the 18S rRNA. Contrary to
previous ideas, transcription, at least in mammals
and Xenopus, proceeds far beyond the end of the
28S rRNA [411].
The Pol I promoters, in contrast to those of
Pol II and Pol III, appear to be taxon specific.
Thus, there is no transcription of mouse rRNA in
extracts of human cells, or of Drosophila virilis
rDNA in extracts of D. melanogaster; however,
NTS
2.8.2 Structure of rRNAs
47
18 S
r--promoters~
(rIl •••••••• ) 1'1
60 / 81 bp -
n = 2_ 6 L--- - -- - - - l
Repeats
- - -- l k b -
two to six segments of 1 kb, made up of repeated elements
of 60 and 81 bp that function as enhancers (black). These
segments contain further promoters [411]
Xenopus laevis and X. borealis are compatible, as
are the mouse and rat, and also man and several
ape species. The reason for this appears to lie
with proteinaceous transcription factors that
show species-specific sequence recognition. However, positive results have been reported with the
rDNA of the beetle Dytiscus in oocytes of the
clawed frog Xenopus, and with Xenopus rDNA in
mouse cells. The efficiency of Pol I-catalysed
transcription is apparently strongly dependent
upon the experimental conditions. Thus, for initiation of transcription in Xenopus the "proximal
promoter", the region between -40 and + 10
(where the initiation site is + 1), is in many cases
sufficient, whereas in other cases optimal transcription requires additional upstream promoters
between -150 and -110. In Xenopus there are further promoters lying 1 or more kb in front of the
initiation site; in addition, there are repetitive
sequences of 60 or 81 bp that act as enhancers to
increase transcription (Fig. 2.15) [411]. The trypanosomes, whose Pol II transcripts with the later
added "spliced leader" sequence are so unusual
(p. 39), apparently have quite normal rDNA transcripts [118].
2.8.2 Structure of rRNAs
All known sequences of SS-rRNAs, LS-rRNAs
and 5.8S rRNAs, together with those of the 5S
rRNAs, tRNAs and snRNAs, are published
regularly in supplementary issues of the journal
Nucleic Acids Research. The length of the SSrRNA in the small ribosomal subunit varies
between 9S (about 640 nt) and 18S (about
1870 nt), and that of the LS-rRNA in the large
subunit varies between 12S (about 1230 nt) and
28S (about 4800 nt). Nevertheless, the primary
and secondary structures of both rRNA classes
are very similar. The differences in length are
the result of insertions in otherwise homologous
regions which do not significantly disturb the
secondary structure of the central region. Open