Abstract Compared to Escherichia coli, the nitrogen- fixing soil cyanobacterium Anabaena sp. strain L-31 exhib- ited significantly superior abilities to survive prolonged and continuous heat stress and recover therefrom. Tempera- ture upshift induced the synthesis of heat-shock proteins of similar molecular mass in the two microbes. However, in Anabaena sp. strain L-31 the heat-shock proteins (par- ticularly the GroEL proteins) were synthesised throughout the stress period, were much more stable and accumulated during heat stress. In contrast, in E. coli the heat-shock proteins were transiently synthesised, quickly turned over and did not accumulate. Nitrogenase activity of Anabaena cells of sp. strain L-31 continuously exposed to heat stress for 7 days rapidly recovered from thermal injury, although growth recovery was delayed. Exposure of E. coli cells to >4.5 h of heat stress resulted in a complete loss of viabil- ity and the ability to recover. Marked differences in the synthesis, stability and accumulation of heat-shock pro- teins appear to distinguish these bacteria in their thermo- tolerance and recovery from heat stress. Keywords Heat shock response · Thermotolerance · Anabaena · Escherichia coli · GroEL Introduction Transient induction of heat-shock proteins (Hsps) in re- sponse to temperature upshift is observed both in prokary- otes and eukaryotes. Most Hsps are synthesised even un- der normal growth conditions, albeit at low rates, and play a fundamental role in cell physiology (Parsell and Lind- quist 1994; Yura et al. 2000). The most abundant of the Hsps in Escherichia coli are either molecular chaperones, like DnaK and GroEL proteins, or proteases, like ClpB and Lon (Yura et al. 2000). The role of the Hsps in the restoration of cellular homeo- stasis and thermotolerance has been a matter of consid- erable research interest. The expression of the heat-in- ducible Hsp70 (DnaK) has been shown to support growth of E. coli, Saccharomyces cerevisiae and Drosophila at moderately high temperatures (40–42 °C), although not at extreme temperatures (50 °C and above) (Parsell and Lind- quist 1994). In E. coli, the DnaK protein has also been found to enable starvation-induced thermotolerance by modulating the levels of the starvation-related sigma fac- tor RpoS (Rockabrand et al. 1998). Similarly, in the clpB mutants of yeast and E. coli, the susceptibility of the cells to extreme temperatures increased and the ability to ac- quire thermotolerance decreased (Parsell and Lindquist 1994; Squires et al. 1991). In the unicellular cyanobacterium Synechococcus sp. strain PCC7942, clpB mutants failed to acquire thermotolerance with respect to photosynthetic parameters such as oxygen evolution (Eriksson and Clarke 1996), but these defects could be complemented by the E. coli clpB gene (Eriksson and Clarke 2000). The GroEL/ES proteins, which act as general chaperones in the folding of proteins in the molten globule state, have been shown to be important for bacterial growth and thermotolerance. Over- expression of GroEL proteins in E. coli ∆rpoH cells, which lack the positive regulator of the heat-shock response (RpoH) and are thermosensitive, permits their growth at temperatures up to 40 °C (Kusukawa and Yura 1988). In contrast to the aforesaid results, which suggest a link- age between the synthesis of certain Hsps and thermotol- erance, in yeast the induction of high levels of Hsps did not correlate with thermotolerance. Mutant cells (hsf1-m3) in- capable of inducing Hsps were found to be as viable as the wild-type yeast cells at 52°C (Smith and Yaffe 1991). Similarly, in E. coli exposure to ethanol or heavy metals caused complete induction of the heat shock regulon with- out the concomitant development of thermotolerance (VanBogelen et al. 1987). Such data diminish the impor- tance of Hsps in thermotolerance. Of particular importance are studies on the thermotol- erance and heat shock response of soil bacteria, such as Hema Rajaram · Shree Kumar Apte Heat-shock response and its contribution to thermotolerance of the nitrogen-fixing cyanobacterium Anabaena sp. strain L-31 Arch Microbiol (2003) 179 : 423–429 DOI 10.1007/s00203-003-0549-0 Received: 10 December 2002 / Revised: 2 April 2003 / Accepted: 8 April 2003 / Published online: 1 May 2003 ORIGINAL PAPER H. Rajaram · S. Kumar Apte (✉) Molecular Biology Division, Bhabha Atomic Research Centre, 400 085 Trombay, Mumbai, India Tel.: +91-22-25595342/25505189, Fax: +91-22-25505326, e-mail: aptesk@apsara.barc.ernet.in © Springer-Verlag 2003