Members of the 70 kD heat-shock gene family are highly conserved across a wide range of organisms. In an effort to learn more about the evolution and possible functions of extreme environment plant Saussurea medusa Maxim hsp70, we isolated a cDNA clone encoding a putative cytosolic member (Smhsp70) of this family of proteins from a cDNA library of S. medusa cell cultures. The cDNA clone was 2224 bp in length and contained a 1941 bp open reading frame (ORF) encoding a polypeptide of 647 amino acid residues with a predicted molecular mass of 70,794 Da. The predicted protein was found to contain a C-terminal amino acid motif of "PKIEEVD" indicating that Smhsp70 was related to cytosolic members of the hsp70 family in higher plant. The secondary and three-dimensional structures of Smhsp70 were analyzed by molecular modeling. The genomic structure of Smhsp70 included one intron of 1134 bp in length. The deduced Smhsp70 protein has 93.7 and 93.2% similarity with the hsp70 of tobacco and tomato, 73.2% with the hsp70 of human, and 43.7% with DnaK of Escherichia. coli, respectively. Semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analyses indicated that the cytosolic Smhsp70 protein was constitutively expressed and markedly increased after relatively short periods of heat shock (37 degrees C) as well as by low temperature (4 degree C) treatments.