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The groESL operon of a psychrophilic bacterium, Colwellia maris, was cloned and sequenced. The operon contains two ORFs of 291 bp and 1,650 bp separated by 210 bp. Northern blot analysis suggested that the groESL operon was transcribed as a bicistronic mRNA, and that the amount of mRNA markedly increased after the temperature was raised from 10 °C to 20 °C. Although the optimum temperatures for GroESL function are different in psychrophilic, mesophilic, and thermophilic bacteria, the deduced amino acid sequences of C. maris GroES and GroEL showed remarkably high similarity with those of GroES and GroEL from mesophilic and thermophilic bacteria. A putative promoter similar to the Escherichia coli σ32 consensus sequence was identified. One specific feature of C. maris groESL was that in the putative untranslated region the G+C content was about 24 mol%, which is much lower than that of mesophilic bacteria such as E. coli. The low G+C content may be important for maintaining transcription at low temperatures.