44
2 Nucleic Acids and Nuclear Proteins
The hsp spectrum differs with the animal species, but in general is neither organ nor development dependent. However, it was recently
observed in Drosophila that the complexity of the
hsp spectrum increases with age: lO-day-old flies
develop 14 hsps and 45-day-old flies develop at
least 59 [138]. In general, two main classes of protein, of 68-110 and 15-30 kDa, can be distinguished, although the types hsp70 and hsp83 usually
dominate; the large hsp100 and hsp110 of the
mammals are not represented in Drosophila.
Both the coding sequences and the control regions of hsp genes are extremely conservative. The
sequence of hsp70 in Drosophila shows 73 %
agreement with the human protein, 72 % with
yeast, and 61 % with the malarial agent Plasmodium falciparum [244, 265]. Hsp83 is just as conservative, showing 63 % agreement between Drosophila and yeast. In many animals there are several hsp70 genes, often with different levels of
inducibility. In Leishmania major, the gene family
includes four members, in Caenorhabditis elegans
there are six, in Drosophila melanogaster five or
six, and in the vertebrates at least two [190, 257,
262]. The heterogeneity of the large hsps may be
further increased by post-translational modification. The spectrum of the small hsps varies greatly
with the species; there are even differences
between different species of Drosophila. In D.
melanogaster one finds in particular hsp28, hsp26,
hsp23 and hsp22; Caenorhabditis elegans has
hsp25, hsp18 and hsp16; and Xenopus laevis and
several lungless salamanders have only hsp30.
The small hsps of a species are all related; in Drosophila, for example, they have approximately
50 % identical amino acids. They are not as
strongly conserved as the large hsps but, nevertheless, show significant agreement between the
insects, nematodes and vertebrates [216, 265,
328].
The hsp70 gene from Drosophila is also heatshock inducible in mouse cells, Xenopus oocytes,
sea-urchin embryos and yeast cells. The control
sequences, the heat-shock elements (HSEs), lie
13-lO0 bp in front of the initiation site and have
the consensus sequence CNNGAANNTTCNNG.
The HSE binds a specific protein (heat-shock
transcription factor, HSTF) that, together with a
further protein, covers about 130 bp of the control region of the hsp70 gene. HSTF is found in
normal cells but is without activity; a heatdependent alteration of the HSTF is apparently
required for its induction. Most hsp genes have
several HSEs, e.g. hsp70 of Drosophila has four,
of which only two are sufficient for maximal
induction [12]. In contrast, at least three HSEs
are required for maximal expression of the Drosophila hsp26 [338]. Accumulation of an hsp causes a negative-feedback inhibition of transcription of the heat-shock gene [265, 328]' Using
gene technology methods, other genes can be
brought under the control of a heat-shock promoter. The hsp27 gene of Drosophila can be induced
by the moulting hormone ecdysone, as well as by
heat shock, using different regulatory elements;
whilst the HSEs lie between bp -270 and -370,
the binding sites for ecdysone receptors are
between bp -579 and -455 [360].
Heat shock in Drosophila causes a 100- to
lOOO-fold increase in transcription of the hsp
genes, whilst the transcription of other genes ceases; simultaneously, the translation of already
available stable mRNAs stops, although they
remain intact and translatable. The hsp mRNAs
differ from other Drosophila mRNAs in having
unusually long "leaders" in front of the coding
region. Introns have been found in only two of
the many hsp genes sequenced so far: hsp83 from
Drosophila and a small hsp from Caenorhabditis.
In contrast to Drosophila, there is no difference
between the translation of hsp mRNAs and other
mRNAs in the mammals; similarly, in the somatic
cells of Xenopus, only the transcription of the
various genes is differentially affected. In the
oocytes, however, the hsp genes are usually
already transcribed and only their translation is
increased following heat shock. It is not known
how these differences arose, but they do allow
some biological interpretation. Drosophila achieves a much quicker heat-shock response through
the simultaneous regulation of transcription and
translation than is either possible or necessary in
the somatic cells of Xenopus; on the other hand,.
the oocytes of Xenopus are so large that an
increase in the transcription rate alone would
require several days to produce an effective heatshock reaction. Intracellular protein degradation
following the binding of proteins to ubiquitin is
intensified following heat shock; in fact, ubiquitin
behaves as a heat-shock-induced gene in the
chicken and also in yeast [265, 328].
2.8 Ribonucleic Acids
and Ribonucleoproteins
Approximately 10 % of the total RNA of the cell
is present in the nucleus; the remaining 90 % is in
2 Nucleic Acids and Nuclear Proteins
The hsp spectrum differs with the animal species, but in general is neither organ nor development dependent. However, it was recently
observed in Drosophila that the complexity of the
hsp spectrum increases with age: lO-day-old flies
develop 14 hsps and 45-day-old flies develop at
least 59 [138]. In general, two main classes of protein, of 68-110 and 15-30 kDa, can be distinguished, although the types hsp70 and hsp83 usually
dominate; the large hsp100 and hsp110 of the
mammals are not represented in Drosophila.
Both the coding sequences and the control regions of hsp genes are extremely conservative. The
sequence of hsp70 in Drosophila shows 73 %
agreement with the human protein, 72 % with
yeast, and 61 % with the malarial agent Plasmodium falciparum [244, 265]. Hsp83 is just as conservative, showing 63 % agreement between Drosophila and yeast. In many animals there are several hsp70 genes, often with different levels of
inducibility. In Leishmania major, the gene family
includes four members, in Caenorhabditis elegans
there are six, in Drosophila melanogaster five or
six, and in the vertebrates at least two [190, 257,
262]. The heterogeneity of the large hsps may be
further increased by post-translational modification. The spectrum of the small hsps varies greatly
with the species; there are even differences
between different species of Drosophila. In D.
melanogaster one finds in particular hsp28, hsp26,
hsp23 and hsp22; Caenorhabditis elegans has
hsp25, hsp18 and hsp16; and Xenopus laevis and
several lungless salamanders have only hsp30.
The small hsps of a species are all related; in Drosophila, for example, they have approximately
50 % identical amino acids. They are not as
strongly conserved as the large hsps but, nevertheless, show significant agreement between the
insects, nematodes and vertebrates [216, 265,
328].
The hsp70 gene from Drosophila is also heatshock inducible in mouse cells, Xenopus oocytes,
sea-urchin embryos and yeast cells. The control
sequences, the heat-shock elements (HSEs), lie
13-lO0 bp in front of the initiation site and have
the consensus sequence CNNGAANNTTCNNG.
The HSE binds a specific protein (heat-shock
transcription factor, HSTF) that, together with a
further protein, covers about 130 bp of the control region of the hsp70 gene. HSTF is found in
normal cells but is without activity; a heatdependent alteration of the HSTF is apparently
required for its induction. Most hsp genes have
several HSEs, e.g. hsp70 of Drosophila has four,
of which only two are sufficient for maximal
induction [12]. In contrast, at least three HSEs
are required for maximal expression of the Drosophila hsp26 [338]. Accumulation of an hsp causes a negative-feedback inhibition of transcription of the heat-shock gene [265, 328]' Using
gene technology methods, other genes can be
brought under the control of a heat-shock promoter. The hsp27 gene of Drosophila can be induced
by the moulting hormone ecdysone, as well as by
heat shock, using different regulatory elements;
whilst the HSEs lie between bp -270 and -370,
the binding sites for ecdysone receptors are
between bp -579 and -455 [360].
Heat shock in Drosophila causes a 100- to
lOOO-fold increase in transcription of the hsp
genes, whilst the transcription of other genes ceases; simultaneously, the translation of already
available stable mRNAs stops, although they
remain intact and translatable. The hsp mRNAs
differ from other Drosophila mRNAs in having
unusually long "leaders" in front of the coding
region. Introns have been found in only two of
the many hsp genes sequenced so far: hsp83 from
Drosophila and a small hsp from Caenorhabditis.
In contrast to Drosophila, there is no difference
between the translation of hsp mRNAs and other
mRNAs in the mammals; similarly, in the somatic
cells of Xenopus, only the transcription of the
various genes is differentially affected. In the
oocytes, however, the hsp genes are usually
already transcribed and only their translation is
increased following heat shock. It is not known
how these differences arose, but they do allow
some biological interpretation. Drosophila achieves a much quicker heat-shock response through
the simultaneous regulation of transcription and
translation than is either possible or necessary in
the somatic cells of Xenopus; on the other hand,.
the oocytes of Xenopus are so large that an
increase in the transcription rate alone would
require several days to produce an effective heatshock reaction. Intracellular protein degradation
following the binding of proteins to ubiquitin is
intensified following heat shock; in fact, ubiquitin
behaves as a heat-shock-induced gene in the
chicken and also in yeast [265, 328].
2.8 Ribonucleic Acids
and Ribonucleoproteins
Approximately 10 % of the total RNA of the cell
is present in the nucleus; the remaining 90 % is in