Stress Genes: Role in Physiological Ecology
hyperthermophiles, temperatures are prone to spatial and temporal variations due to tidal
flexing of the earth’s crust which causes diurnal temperature fluctuations [42], dynamic
fluid flow patterns, and steep temperature gradients. Therefore, hyperthermophiles must
employ thermal stress mechanisms to withstand the super-optimal temperatures encountered
from these variations.
In vivo
evidence for heat shock in hyperthermophiles
When a culture of microorganisms is exposed to super-optimal growth temperatures, the
number of viable cells decreases exponentially with time, and this rate of decline increases
exponentially with increasing temperature [32]. However, if a culture is exposed to a mild
hyperthermal stress temperature prior to exposure to a more lethal temperature, then the
number of viable cells in the culture will remain significantly higher for a period of time
before they begin to die. This kinetic display of enhanced tolerance to super-optimal
temperatures is known as ‘acquired thermotolerance’ and is attributed to the expression of
the heat-shock response [33]. Acquired thermotolerance kinetics have been observed in the
sulfur-oxidizing hyperthermophile
Sulfolobus shibatae
and in the anaerobic sulfur reducer
Pyrococcus
sp. strain ES4 [13,47]. ES4 grown at 95°C (optimum growth at 99°C)
demonstrated acquired thermotolerance kinetics at 105°C when cultures were exposed to
102°C for 90 min prior to the shift to 105°C [13].
In both
S. shibatae
and ES4, proteins were produced during exposure to a mild super-
optimal temperature, though the overall rate of protein synthesis decreased [47, J Holden
and J Baross, unpublished results]. The densest protein band produced in both organisms
during heat-shock, as seen by pulse labeling with
35
S-labeled amino acids, had a mass of
approximately 60 kDa and was shown to be a chaperonin [see below]. In ES4, other
proteins of various masses were also produced whose function remains unknown.
Furthermore, during
in vivo
exposure to 102°C for up to 90 min, six metabolic proteins
maintained constant specific activity [J Holden and MWW Adams, unpublished results]
demonstrating that some factor(s), extrinsic or intrinsic, was protecting these proteins at the
super-optimal temperature for this organism.
Hyperthermophilic chaperones
The most abundant protein produced during the heat shock response of hyperthermophiles
is the TF55 chaperone. This well-studied enzyme is composed of two stacked rings made
of either eight or nine protein subunits per ring. The enzyme is a hexadecamer in
Pyrodictium occultum
[34,35] and an octadecamer in
S. shibatae
[25,46], these are
heteromeric proteins, and consist of one of two closely related proteins, each with a mass of
approximately 60 kDa, which are present in a 1:1 ratio [16,35]. This chaperone is
expressed constitutively and is abundant in hyperthermophiles under normal growth
conditions, suggesting that it is used to protect proteins (perhaps intermediates formed
during translation) from high temperatures when the cell is not under duress. The TF55
chaperone, a homolog of the enzyme (as detected by antibodies), or a homolog of the TF55
gene, has been found in all of the major genera of hyperthermophilic archaea
[2,16,17,34,48]. The chaperone from
Sulfolobus
spp. reduced the denaturation rate of a
target protein at high temperatures [46] and hydrolyzed ATP in the presence of K
+
[10,20].
The chaperone from
S. solfataricus
underwent a major conformational change in the
presence of ATP and Mg
2+
, bound to denatured target protein with refolding occurring
