Distinct cyclin D genes show mitotic accumulation or constant levels of transcripts in tobacco bright yellow-2 cells

The commitment of eukaryotic cells to division normally occurs during the G 1 phase of the cell cycle. In mammals D-type cyclins regulate the progressi on of cells through G1 and therefore are important for both proliferative a nd developmental controls. Plant CycDs (D-type cyclin homologs) have been i dentified, but their precise function during the plant cell cycle is unknow n. We have isolated three tobacco (Nicotiana tabacum) CycD cyclin cDNAs: tw o belong to the CycD3 class (Nicta;CycD3;1 and Nicta;CycD3;2) and the third to the CycD2 class (Nicta; CycD2;1). To uncouple their cell-cycle regulati on from developmental control, we have used the highly synchronizable tobac co cultivar Bright Yellow-2 in a cell-suspension culture to characterize ch anges in CycD transcript levels during the cell cycle. In cells re-entering the cell cycle from stationary phase, CycD3;2 was induced in G1 but subseq uently remained at a constant level in synchronous cells. This expression p attern is consistent with a role for CycD3;2, similar to mammalian D-type c yclins. In contrast, CycD2;1 and CycD3;1 transcripts accumulated during mit osis in synchronous cells, a pattern of expression not normally associated with D-type cyclins. This could suggest a novel role for plant D-type cycli ns during mitosis.