2 ARABIDOPSIS CYCLIN PROMOTERS MEDIATE DISTINCTIVE TRANSCRIPTIONAL OSCILLATION IN SYNCHRONIZED TOBACCO BY-2 CELLS

Cyclins are cell cycle regulators whose proteins oscillate dramaticall y during the cell cycle. Cyclin steady-state mRNA levels also fluctuat e, and there are indications that both their rate of transcription and mRNA stability are under cell cycle control. Here, we demonstrate the transcriptional regulation of higher eukaryote cyclins throughout the whole cell cycle with a high temporal resolution, The promoters of tw o Arabidopsis cyclins, cyc3aAt and cyc1At, mediated transcriptional os cillation ofthe beta-glucuronidase (gus) reporter gene in stably trans formed tobacco BY-2 cell lines. The rate of transcription driven by th e cyc3aAt promoter was very low during G(1), slowly increased during t he S phase, peaked at the G(2) phase and G(2)-to-M transition, and was down-regulated before early metaphase. In contrast, the rate of the c yc1At-related transcription increased upon exit of the S phase, peaked at the G(2)-to-M transition and during mitosis, and decreased upon ex it from the M phase. This study indicates that transcription mechanism s that seem to be conserved among species play a significant role in r egulating the mRNA abundance of the plant cyclins. Furthermore, the tr anscription patterns of cyc3aAt and cyc1At were coherent with their sl ightly higher sequence similarity to the A and B groups of animal cycl ins, respectively, suggesting that they may fulfill comparable roles d uring the cell cycle.