SummarySynchronously dividing cell cultures of Catharanthus roseus were used to isolate cDNAs for two mitotic cyclins, named CYS and CYM. The deduced protein sequence of CYS is similar to that of A-type cyclins, and CYM 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 early in the G2 phase, whereas CYM mRNA was detected in the G2 and M phases of the cell cycle. Tobacco homologs of the two genes showed similar 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 along with the soybean cyclGm can be classified into a distinct subclass. The activities of CYMand CYS promoters during the cell cycle were analyzed in stably transformed tobacco BY2 cells. Cyclin promoter sequences of 0.5 kb could confer the typical cell-cycle-dependent expression to the ~-glucuronidase (GUS) reporter gene: the CYS Received 15 July 1996; revised 20 December 1996; accepted 17 January 1997. *For correspondence (fax +81 3 3814 1728; e-mail masakito@uts2.s.u-tokyo.ac.jp). tPresent address: Laboratoire de Biologie Generale, CNRS Universite Louis Pasteur, 12 Rue de I'Universite, 67000 Strasbourg, France. tPresent address: Botanical Gardens, Faculty of Science, University of Tokyo, Hakusan, Tokyo, 112 Japan.promoter directed S-phase-specific expression, whereas the CYM promoter drove M-phase-specific expression. These results indicate the important role of transcriptional regulation in the oscillations of cyclin mRNA levels during the cell cycle.
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