The heat shock response of the psychrophilic bacterium Pseudoalteromonas haloplanktis TAC 125 (PhTAC 125) gives rise to the production of several inducible proteins. Among these, the protein corresponding to a 55-kDa band on SDS-PAGE was purified to homogeneity and identified as a GroEL-like protein. The gene coding for this protein (PhGroEL) was cloned and sequenced; the deduced amino acid sequence shows 82% sequence identity to GroEL from Escherichia coli (EcGroEL). The ORF found in the 5' upstream region codes for a homologue of the GroES from E. coli (PhGroES, 71% sequence identity to EcGroES). PhGroEL shows a chaperone activity and can use GroES from E. coli as a co-chaperone. PhGroEL melting temperature, 6 degrees C lower than that of EcGroEL, and equilibrium unfolding experiments in urea showed a less stable protein architecture for the psychrophilic GroEL. The data herein reported demonstrate that PhGroEL cold adaptation consists in a shift of the protein properties toward lower temperatures without increasing catalytic efficiency at low temperatures. Primary extension analysis depicted a complex organization of regulative elements for the operon containing the genes coding for PhgroES and PhgroEL (PhgroE), suggesting that a fine-tuning of transcription can also be involved in thermal adaptation of PhTAC 125.