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 cha racteristic transcription pattern of mRNAs encoding ubiquitin extensio n proteins [0.7-kb transcript(s)] as well as polyubiquitin (2.3-kb tra nscript). Heat shock resulted in an increase in the amount of polyubiq uitin mRNA (up to tenfold compared with control cells). Chilling in th e 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 wa s observed after applying either chilling in darkness or photoinhibiti on. The latter stress, however, led to a dramatic decrease in mRNAs en coding ubiquitin extension proteins (down to 23% of control cells). Ex periments 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 th e assumption that the expression of the corresponding gene is necessar y for completion of the C. reinhardtii vegetative cell cycle.