Enhanced thermotolerance and temperature-induced changes in protein composition in the hyperthermophilic archaeon ES4

Abstract

The hyperthermophilic archaeon ES4, a heterotrophic sulfur reducer isolated from a deep-sea hydrothermal vent, is capable of protecting itself from thermal stress at temperatures above its optimum for growth. The thermotolerance of ES4 was determined by exposing log-phase cells to various lethal high temperatures. When ES4 was shifted from 95 to 102'C, it displayed recovery from an exponential rate of death, followed by transient thermotolerance. When All organisms examined thus far, from Escherichia coli to mammals, contain a stress response mechanism known as the heat shock response (6, 9). The heat shock response involves the increased synthesis of proteins which protect an organism from thermal and other stresses. In general, the primary function of heat shock proteins is to stabilize, repair, or degrade proteins damaged by thermal stress and to prepare for a return to normal temperatures (6, 9). Heat shock is a transient response to exposure to temperatures above the optimum for growth, or hyperthermal conditions. A heat-shocked organism cannot carry out hyperthermal growth indefinitely (9, 24); thus, heat shock is a means of "weathering" a period of thermal stress until a return to normal temperatures occurs.There are organisms capable of growth at temperatures above the mesophilic temperature range. These organisms are the thermophiles and the hyperthermophiles, which grow at temperatures of above 60 and 90°C, respectively (2).Although thermophiles are found among both bacteria and archaea, the hyperthermophiles are highly represented among the archaea (25). Presently, our understanding of how archaea grow and survive at temperatures above 100°C is very incomplete. One postulated mechanism for high-temperature survival of archaea is the presence of tetraether lipids, whose cyclic structure forms a bonded cell wall bilayer (5, 19). Other mechanisms involve reverse gyrase, which creates a positive supercoil in primarily hyperthermophilic DNA (3), and the histone HMf, which adds thermotolerance to double-stranded DNA (18).Another mechanism for survival among hyperthermophilic archaea is a heat shock-like response found in Sulfolobus shibatae and Pyrodictium occultum (12, 23). Unlike heat shock in mesophilic organisms, these archaea constitutively produce a pair of proteins which remain present over the entire temperature growth range for each organism (12,22,23). Furthermore, the relative abundance of all other pro-* Corresponding author. teins decreases significantly under hyperthermal conditions. In both organisms, these two proteins associate into a complex which is capable of binding to denatured proteins.This research sought to determine whether a heat shocklike response exists in ES4, a heterotrophic sulfur-reducing archaeon isolated from a hydrothermal vent structure (13). Two experimental approaches were used. First, thermal death-curve experiments were run to determine whether enhanced thermotolerance occurs in ES4 when it is exposed to lethal high temperatures. Second, the protein composition of ...

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