CELL-CYCLE-REGULATED TRANSCRIPTION OF A-TYPE AND B-TYPE PLANT CYCLIN GENES IN SYNCHRONOUS CULTURES

Synchronously dividing cell cultures of Catharanthus roseus were used to isolate cDNAs for two mitotic cyclins, named CYS and CYM. The deduc ed protein sequence of CYS is similar to that of A-type cyclins, and C YM belongs to the group of B-type cyclins. In a fashion similar to the pattern of expression seen for A-type and B-type cyclins in mammalian cells, CYS is expressed before CYM in C. roseus cells during the cell cycle. CYS mRNA accumulated at the onset of S phase and disappeared e arly in the G2 phase, whereas CYM mRNA was detected in the G2 and M ph ases of the cell cycle. Tobacco homologs of the two genes showed simil ar cell-cycle dependent expression patterns in synchronous cultures of tobacco BY2 cells. In both systems, CYS was expressed much earlier in the cell cycle than most other plant A-type cyclins, and hence CYS al ong with the soybean cyc1Gm can be classified into a distinct subclass . The activities of CYM and CYS promoters during the cell cycle were a nalyzed in stably transformed tobacco BY2 cells. Cyclin promoter seque nces of 0.5 kb could confer the typical cell-cycle-dependent expressio n to the beta-glucuronidase (GUS) reporter gene: the CYS promoter dire cted S-phase-specific expression, whereas the CYM promoter drove M-pha se-specific expression. These results indicate the important role of t ranscriptional regulation in the oscillations of cyclin mRNA levels du ring the cell cycle.