The Heat-Shock Response 
our investigations of the regulation of the re- 
sponse have concentrated on post-transcriptional 
mechanisms that are employed to maximize the 
synthesis of the HSPs during heat shock or to shut 
off the synthesis of HSPs after heat shock. Tom 
McGarry and Bob Petersen have found that heat- 
shock mRNAs in Drosophila cells are preferen- 
tially translated during heat shock by virtue of 
sequences in their 5'-untranslated leaders. They 
are preferentially repressed after heat shock 
(during recovery) by virtue of sequences in their 
3'-untranslated tails, sequences that selectively 
target them for destruction. The latter sequences 
are shared by many non-heat-shock messages, 
which have the common property of being rap- 
idly degraded at normal temperatures. Heat- 
shock regulation takes advantage of this common 
pathway to control HSP expression. The mecha- 
nism is inactivated during heat shock and re- 
stored during recovery. 
Joseph Yost demonstrated that heat shock 
blocks the processing of messenger RNA precur- 
sors, explaining the puzzling observation that 
heat-shock genes generally do not have interven- 
ing sequences. In this line of research our studies 
of HSP function and regulation overlap. Mild pre- 
heat treatments, which induce the synthesis of 
HSPs, protect RNA processing from disruption 
during heat shock. Examination of yeast cells 
carrying mutations in various HSP genes sug- 
gests that both the HSP 104 and the HSPIO genes 
play a role in protecting processing at high 
temperatures. 
To investigate the function of the HSPs, we 
have created a series of mutations in the genes of 
both yeast and Drosophila. Kathy Borkovich 
found that hsp83 is essential for growth at all tem- 
peratures in yeast cells, but that it is required at 
higher concentrations for growth at higher tem- 
peratures. Thus induction of this protein is re- 
quired for cells to grow at the upper end of their 
normal temperature range. We believe the pro- 
tein is needed to regulate the activity of a wide 
variety of other cellular proteins and that it is 
needed at higher concentrations at high tempera- 
tures to drive the equilibrium of these interac- 
tions toward complex formation. Recently, in col- 
laboration with Keith Yamamoto's laboratory, we 
have demonstrated that hsp82 interacts with the 
steroid hormone family of receptors and helps 
these proteins fold into an active conformation. 
Yolanda Sanchez created mutations in the 
HSP 104 gene of yeast. The mutations have no ef- 
fect on growth at high or low temperatures. How- 
ever, the cells are unable to acquire tolerance to 
extreme temperatures when given a mild preheat 
treatment. Thus this mutation confirms the long- 
standing assumption that HSPs play a vital role in 
establishing thermotolerance. More recently, we 
have found that this protein is highly conserved 
in mammals and in prokaryotic cells. Moreover, it 
appears to provide protection from many other 
forms of stress, such as exposure to ethanol and 
sodium arsenite. 
In Drosophila our mutational analysis has con- 
centrated on hsp70. Jan Rossi found that varying 
the level of hsp70 expression in Drosophila cells 
varies the rate at which cells recover from heat 
shock. Jonathan Solomon found that expressing 
hsp70 from independently regulated promoters, 
in the absence of heat shock, helps cells to sur- 
vive extreme temperatures but inhibits their 
growth. This particular protein thus helps to pro- 
tect cells from the ravages of extreme tempera- 
tures but is disadvantageous at normal tempera- 
tures. Further experiments will focus on 
examining the specific cellular and developmen- 
tal processes in which hsp70 plays a role. 
Finally, Kent Golic has taken advantage of the 
heat-shock response to devise an inducible sys- 
tem for site-specific recombination in Drosoph- 
ila. He placed the FLP recombinase gene of yeast 
under the control of the hsp70 regulatory se- 
quences and transformed Drosophila embryos 
with this construct. He transformed other em- 
bryos with a recombinase target, consisting of an 
eye color gene flanked by target sequences for 
FLP recombination. When the two strains were 
crossed and their progeny were heat-shocked, the 
eye color gene was excised in both somatic and 
germline tissues, with the frequency of excision 
depending on the severity of the heat shock. This 
system is being used in our laboratory to vary the 
dosage of hsp mutants, but it has many other po- 
tential applications in Drosophila and other 
organisms. 
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