DIFFERENTIAL EXPRESSION OF A CAK (CDC2-ACTIVATING KINASE)-LIKE PROTEIN-KINASE, CYCLINS AND CDC2 GENES FROM RICE DURING THE CELL-CYCLE AND IN RESPONSE TO GIBBERELLIN

Progress through the eukaryotic cell cycle is regulated by cyclin-depe ndent cdc2 protein kinases. In rice (Oryza sativa L.), two cdc2 protei n kinases, cdc2Os-1 and cdc2Os-2, and two cyclins, cycOs1 and cycOs2, have been described. In this study, we report on the cell-cycle phase- specific expression of these genes. Using partially synchronized suspe nsion cells from rice, we found that cdc2Os-1 was expressed constituti vely throughout the cell cycle. The cdc2Os-2 transcript level was elev ated in G1 and S phase. The cycOs1 and cycOs2 transcripts increased st eadily through G2 and dropped off rapidly in mitosis as is typical for mitotic cyclins. We hypothesize that the cdc2Os-2 gene product acts i n G1/S and that the growth-promoting hormone gibberellin (GA) that ind uces expression of cdc2Os-2, cycOs1 and cycOs2 in the intercalary meri stem of deepwater rice internodes accelerates G1/S phase progression t hrough increased expression of cdc2Os-2 and G2/M phase progression thr ough increased expression of the mitotic cyclins cycOs1 and cycOs2. Th e R2 gene from rice has 55% sequence identity to the cdc2-activating k inase (CAK) family of protein kinases which have been shown to phospho rylate and thereby activate cdc2 protein kinases in animals and yeast. In partially synchronized suspension cells, R2 mRNA levels were eleva ted in G1 and S phase. In GA-treated rice internodes, R2 transcript le vels were elevated in the meristem and part of the elongation zone. Th ese results are consistent with a role for R2 in regulating G1/S phase progression.