CycMs3, a novel B-type alfalfa cyclin gene, is induced in the G₀-to-G₁ transition of the cell cycle

Cyclins are key regulators of the cell cycle in all eukaryotes. We have previously isolated two B-type cyclin genes, cycMs1 and cycMs2, from alfalfa that are primarily expressed during the G₂-to-M phase transition and are most likely mitotic cyclin genes. Here, we report the isolation of a novel alfalfa cyclin gene, termed cycMs3 (for cyclin Medicago sativa), by selecting for mating type α-pheromone-induced cell cycle arrest suppression in yeast. The central region of the predicted amino acid sequence of the cycMs3 gene is most similar to the cyclin box of yeast B-type and mammalian A- and B-type cyclins. In situ hybridization showed that cycMs3 mRNA can be detected only in proliferating cells and not in differentiated alfalfa cells. When differentiated G₀-arrested cells were induced to reenter the cell cycle in the G₁ phase and resume cell division by treatment with plant hormones, cycMs3 transcript levels increased long before the onset of DNA synthesis. In contrast, histone H3-1 mRNA and cycMs2 transcripts were not observed before DNA replication and mitosis, respectively. In addition, cycMs3 mRNA was found in all stages of the cell cycle in synchronously dividing cells, whereas the cycMs2 and histone H3-1 genes showed a G₂-to-M phase- or S phase-specific transcription pattern, respectively. These data suggest that the role of cyclin CycMs3 differs from that of CycMs1 and CycMs2. We propose that CycMs3 helps control reentry of quiescent G₀-arrested cells into the G₁ phase of the cell cycle.