RUBISCO ADAPTATION TO LOW-TEMPERATURES - A COMPARATIVE-STUDY IN PSYCHROPHILIC AND MESOPHILIC UNICELLULAR ALGAE

Some properties of the ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) from two psychrophilic Chloromonas species have been invest igated in relation to their adaptation to cold environments. Contrary to the situation usually encountered with psychrophilic enzymes, the c arboxylase activity of both purified ''cold'' RUBISCO enzymes was lowe r at low temperatures than that found with the enzyme of the mesophili c alga Chlamydomonas reinhardtii Dangeard. Moreover, the apparent opti mal temperature for RUBISCO carboxylase activity was similar for psych rophilic and mesophilic enzymes. Psychrophilic RUBISCOs, however, show ed a greater thermosensitivity than the C. reinhardtii enzyme. Genes e ncoding small and large subunits of RUBISCO from one psychrophilic iso late were sequenced. Comparison of the deduced amino acid sequences to those of higher plants and green algae revealed the substitution of a very highly conserved residue (cystein(247) --> serine in the large s ubunit) that could be responsible, at least in part, for the increased thermosensitivity of the ''cold'' enzyme. Interestingly, the relative amount of RUBISCO subunits found in the psychrophilic isolates was ab out twice as high as the amount observed in C. reinhardtii and five ot her mesophilic algae. The high production of a hey enzyme to counterba lance its poor catalytic efficiency at low temperature could constitut e a novel type of adaptive mechanism to cold environments.