THE D-TYPE ALFALFA CYCLIN GENE CYCMS4 COMPLEMENTS G(1) CYCLIN-DEFICIENT YEAST AND IS INDUCED IN THE G(1) PHASE OF THE CELL-CYCLE

Cyclins are key regulators of the cell cycle in all eukaryotes. In alf alfa, we have previously isolated three B-type cyclins. The closely re lated cycMs1 and cycMs2 genes are expressed primarily during the G(2) and M phases and are most likely mitotic cyclins; expression of the cy cMs3 gene is induced in the G(0)-to-G(1) transition, when cells reente r the cell cycle. By complementation of G(1) cyclin-deficient yeast ce lls, a novel alfalfa cyclin, designated cycMs4, was isolated, The pred icted amino acid sequence of the cycMs4 gene is most similar to that o f the Arabidopsis cyclin delta 3 gene. CycMs4 and cyclin delta 3 belon g to the class of D-type cyclins and contain PEST-rich regions and a r etinoblastoma binding motif, When comparing expression levels in diffe rent organs, cycMs4 transcripts were present predominantly in roots, W hereas expression of the cycMs4 gene was cell cycle-regulated in suspe nsion-cultured cells, transcription in roots was observed to depend al so on the positional context of the cell. When differentiated G(0)-arr ested leaf cells were induced to resume cell division by treatment wit h plant hormones, cycMs4 transcription was induced before the onset of DNA synthesis, Whereas this induction was preceded by that of the cyc Ms3 gene, cycMs2 expression occurred later and at the same time as mit otic activity, These data suggest that cycMs4 plays a role in the G(1) -to-S transition and provide a model to investigate the plant cell cyc le at the molecular level.