CYCMS3, A NOVEL B-TYPE ALFALFA CYCLIN GENE, IS INDUCED IN THE G(0)-TO-G(1) TRANSITION OF THE CELL-CYCLE

Cyclins are key regulators of the cell cycle in all eukaryotes. We hav e previously isolated two B-type cyclin genes, cycMs1 and cycMs2, from alfalfa that are primarily expressed during the G(2)-to-M phase trans ition and are most likely mitotic cyclin genes. Here, we report the is olation of a novel alfalfa cyclin gene, termed cycMs3 (for cyclin Medi cago sativa), by selecting for mating type a-pheromone-induced cell cy cle arrest suppression in yeast. The central region of the predicted a mino acid sequence of the cycMs3 gene is most similar to the cyclin bo x of yeast B-type and mammalian A- and B-type cyclins. in situ hybridi zation showed that cycMs3 mRNA can be detected only in proliferating c ells and not in differentiated alfalfa cells. When differentiated G(0) -arrested cells were induced to reenter the cell cycle in the G(1) pha se and resume cell division by treatment with plant hormones, cycMs3 t ranscript levels increased long before the onset of DNA synthesis. In contrast, histone H3-1 mRNA and cycMs2 transcripts were not observed b efore DNA replication and mitosis, respectively. In addition, cycMs3 m RNA was found in all stages of the cell cycle in synchronously dividin g cells, whereas the cycMs2 and histone H3-1 genes showed a G(2)-to-M phase- or S phase-specific transcription pattern, respectively. These data suggest that the role of cyclin CycMs3 differs from that of CycMs 1 and CycMs2. We propose that CycMs3 helps control reentry of quiescen t G(0)-arrested cells into the G(1) phase of the cell cycle.