Abstract
Cyclin B genes are specifically expressed at the G2/M phase in the plant cell cycle. c-myb-like transcription factors, NtmybA1 and NtmybA2, were identified as transcription activators which regulate cell cylce-dependent cyclin B transcription. Here we showed that the activity of NtmybA2 for transactivation is controlled at the post-translational level. In co-transfection experiments, transactivation activity of NtmybA2 was enhanced by coexpression of cyclin B. Mutation at the potential phosphorylation sites for CDK in NtmybA2 blocked the enhancing effect of the cyclin on the NtmybA2 activity. Recombinant NtmybA2 was phosphorylated in vitro by a CDK franction prepared from tobacco BY2 cells. The kinase acrivity for NtmybA2 in the CDK fraction was cell cycle-regulated, peaking at the G2/M phase. Our data suggest that activity of NtmybA2 is enhanced by direct phosphorylation of NtmybA2 by CDK at the G2/M, thereby facilitating G2/M phase-specific transcription of cyclin B genes.
