Cyclin-dependent kinases (CDKs) control the key transitions in the eukaryotic cell cycle. All the CDKs known to control G 2 /M progression in yeast and animals are distinguished by the characteristic PSTAIRE motif in their cyclin-binding domain and are closely related. Higher plants contain in addition a number of more divergent non-PSTAIRE CDKs with still obscure functions. We show that a plant-specific type of non-PSTAIRE CDKs is involved in the control of the G 2 /M progression. In synchronized tobacco BY-2 cells, the corresponding protein, accumulated in a cell cycle-regulated fashion, peaking at the G 2 /M transition. The associated histone H1 kinase activity reached a maximum in mitosis and required a yet unidentified subunit to be fully active. Down-regulation of the associated kinase activity in transgenic tobacco plants using a dominantnegative mutation delayed G 2 /M transition. These results provide the first evidence that non-PSTAIRE CDKs are involved in the control of the G 2 /M progression in plants.
Cell cycle progression is regulated by cyclin-dependent kinases (CDKs). Arabidopsis thaliana contains two cdk genes, cdc2aAt and cdc2bAt. This paper compares the developmental and cell cycle phase-dependent transcription of both cdk genes. In situ hybridizations revealed that cdc2bAt steady-state mRNAs, much like cdc2aAt, are found both in meristematic cells and cells with a high proliferative competence. Cdc2aAt is expressed in every meristematic cell whereas cdc2bAt is found to be expressed in a patchy pattern. An even smaller number of meristematic cells express the mitotic cyc1At. These data indicate that cdc2bAt and cyc1At mRNAs accumulate in a particular cell cycle phase in agreement with evidence provided by hybridization experiments of flow cytometrysorted nuclei and the use of cell cycle blockers on roots. The data indicate that cdc2bAt is preferentially expressed in S and G2 phases whereas cdc2aAt expression is constitutive throughout the cell cycle, as shown previously. The existence of two distinct CDK classes in plants is proposed: (i) constitutively expressed CDKs containing a PSTAIRE motif (e.g. cdc2aAt) and (ii) CDKs with divergent motifs which are expressed during a limited interval of the cell cycle (e.g. cdc2bAt).
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