EXPRESSION OF UBIQUITIN GENES IN CHLAMYDOMONAS-REINHARDTII - INVOLVEMENT IN STRESS-RESPONSE AND CELL-CYCLE

Applying different stresses (heat shock, photoinhibition, and chilling) to the unicellular green alga Chlamydomonas reinhardtii led to a characteristic transcription pattern of mRNAs encoding ubiquitin extension proteins [0.7-kb transcript(s)] as well as polyubiquitin (2.3-kb transcript). Heat shock resulted in an increase in the amount of polyubiquitin mRNA (up to tenfold compared with control cells). Chilling in the light led to a sevenfold increase in the amount of 2.3-kb transcript and to a twofold increase in the amount of 0.7-kb mRNA(s), whereas a less pronounced effect on the level of the polyubiquitin transcript was observed after applying either chilling in darkness or photoinhibition. The latter stress, however, led to a dramatic decrease in mRNAs encoding ubiquitin extension proteins (down to 23% of control cells). Experiments performed with a temperature-sensitive cell-cycle mutant of C. reinhardtii showed that the 2.3-kb polyubiquitin mRNA was no longer transcribed when cells were shifted to a non-permissive temperature, and thus were blocked in their vegetative cell cycle. This leads to the assumption that the expression of the corresponding gene is necessary for completion of the C. reinhardtii vegetative cell cycle.