UBIQUITIN METABOLISM IN CHLAMYDOMONAS-REINHARDTII FOLLOWING COLD SHOCK

The present work characterizes parameters of ubiquitin turnover in Chl amydomonas reinhardtii Dangeard growing under constant temperature con ditions and after an exposure to cold shock. The ratio of free and con jugated ubiquitin to total protein and the rate constant of ubiquitin synthesis and conjugation increased about 2-fold during the first 4 h after cold treatment, whereas the rate constant of ubiquitin degradati on reached its maximum 8 h after treatment. The half-life of ubiquitin calculated from the constant of degradation decreased from 6 h to 3.5 h during the first 4 h after completion of the cold treatment. The ra te constant of ubiquitin deconjugation did not change after cold treat ment. The ratio of free to conjugated ubiquitin decreased temporarily to approximately 8 immediately after cold treatment and increased back to its original value of about 11 at 2 h after cold treatment. These observations raise questions regarding the regulatory mechanisms of ub iquitin synthesis and degradation.