PETUNIA P34(CDC2) PROTEIN-KINASE ACTIVITY IN G(2)M CELLS OBTAINED WITH A REVERSIBLE CELL-CYCLE INHIBITOR, MIMOSINE

Protoplasts isolated from petunia leaf mesophyll are non-cycling cells mostly with 2C content. Cells regenerating from protoplast culture en ter mitosis after 48 h. This experimental model is used to relate p34c dc2 kinase activity to cell cycle phase. Our results show that the his tone H1 phosphorylation, and hence p34cdc2 kinase activity, peaks with G2+early M cell cycle phase. However, a trace kinase activity was alr eady present when most cells were entering S phase. To obtain a maximu m of cells in G1+S phases, the protoplast culture was treated with the rare amino acid, mimosine. Mimosine blocked plant cells derived from protoplast culture both at G1 and in early and mid S phase. Despite th e increased G1+S level, p34cdc2 kinase activity did not increase. This suggests that the trace activity appearing when the majority of cells are entering S does not correspond to any putative p34cdc2 activation at G1/S transition but to the activation of the minor 4C population i nitially present in the leaf: the hypothesis remains that p34cdc2 kina se activity is solely related to G2+M phase in petunia.