The Heat-Shock Response 
Heat-shock regulation takes advantage of this 
common pathway to control HSP expression. The 
mechanism is inactivated during heat shock and 
restored during recovery. 
Joseph Yost demonstrated that heat shock 
blocks the processing of mRNA precursors, 
which explains why heat-shock genes generally 
do not have intervening sequences. (If they had, 
the block in splicing would prevent expression.) 
Sudden high-temperature heat shocks also inhibit 
transcription termination (discovered by Robert 
Dellavalle). By some mechanism we do not yet 
understand, heat-shock genes are more likely to 
be terminated correctly than normal cellular 
genes. 
Although our studies of HSP function are in 
many ways independent of our studies on regula- 
tion, in one important respect they overlap. HSPs, 
and hsp70 in particular, play an important role in 
restoring normal gene expression patterns after 
heat shock. They are required at the level of 
translation, RNA turnover, RNA processing, and 
transcription. 
To investigate the function of the HSPs, we cre- 
ated a series of mutations in the genes of both 
yeast and Drosophila. Kathy Borkovich found 
that hsp82 is essential for growth at all tempera- 
tures in yeast cells, but is required at higher con- 
centrations for growth at high temperatures. Thus 
induction is required for cells to grow at the up- 
per end of their normal temperature range. We 
believe the protein is needed to regulate the activ- 
ity of a wide variety of other cellular proteins and 
that it is needed at higher concentrations at high 
temperatures in order to drive the equilibrium of 
these interactions toward complex formation. 
In collaboration with Keith Yamamoto's labora- 
tory, Bushra Khursheed and Marc Fortin demon- 
strated that hsp82 interacts with the steroid hor- 
mone family of receptors and helps these 
proteins fold into an active conformation. Most 
recently, Yang Xu found that hsp82 is also re- 
quired for the maturation of oncogenic proteins 
in the src family. 
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. Moreover, it pro- 
vides protection from many other forms of stress, 
such as exposure to ethanol and sodium arsen- 
ite. Dawn Parsell found that hspl04 is highly 
conserved in mammals and in prokaryotic cells 
and contains two essential nucleotide-binding 
domains. 
In Drosophila our mutational analysis has con- 
centrated on hsp70. Janice Rossi found that vary- 
ing the level of hsp70 expression in Drosophila 
cells varies the rate at which the cells recover 
from heat shock. Jonathan Solomon found that 
expressing hsp70 from independently regulated 
promoters, in the absence of heat shock, helps 
cells to survive extreme temperatures but in- 
hibits their growth. Thus hsp70 helps to protect 
cells from the ravages of extreme temperatures 
but is actually disadvantageous at normal 
temperatures. 
To study the role of hsp70 in whole flies, Kent 
Golic developed a new system for manipulating 
the Drosophila genome. He created flies that ex- 
press the FLP recombinase gene of yeast under 
the control of heat-shock regulatory sequences. 
When flies that also carry a recombinase target 
sequence are given a very mild heat shock, the 
recombinase is induced and catalyzes rearrange- 
ments of the target sequence. 
Michael Welte and Joan Tetrault have em- 
ployed this site-specific recombination system to 
study the role of hsp70 in induced thermotoler- 
ance. Strains that carry several extra copies of the 
hsp70 gene were constructed. Embryos from 
these lines survive heat treatments much better 
than wild-type embryos. This suggests that it will 
be possible to alter the stress tolerance of even 
developmentally complex species. 
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