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 investigated in relation to their adaptation to cold environments. Contrary to the situation usually encountered with psychrophilic enzymes, the carboxylase activity of both purified "cold" RUBISCO enzymes was lower at low temperatures than that found with the enzyme of the mesophilic alga Chlamydomonas reinhardtii Dangeard. Moreover, the apparent optimal temperature for RUBISCO carboxylase activity was similar for psychrophilic and mesophilic enzymes. Psychrophilic RUBISCOs, however, showed a greater thermosensitivity than the C. reinhardtii enzyme. Genes encoding small and large subunits of RUBISCO from one psychrophilic isolate 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 subunit) 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 about twice as high as the amount observed in C. reinhardtii and five other mesophilic algae. The high production of a hey enzyme to counterbalance its poor catalytic efficiency at low temperature could constitute a novel type of adaptive mechanism to cold environments.