Mitotic cyclins stimulate the activity of c-Myb-like factors for transactivation of G2/M phase-specific genes in tobacco.
Myb transcription factors, which contain three imperfect repeats in the Myb domain, are evolutionarily conserved members of the Myb superfamily. Vertebrate Myb proteins with three repeats, c-Myb, A-Myb, and BMyb, play important roles at the G(1)/S transition in the cell cycle. In plants, this type of Myb protein controls the G(2)/M phase by activating or repressing the transcription of cyclin B genes and a variety of other G(2)/M phase-specific genes. In tobacco, two genes for Myb activators, NtmybA1 and NtmybA2, are transcriptionally controlled and are expressed specifically at the G(2)/M phase. As we showed here, in addition to the control at the transcriptional level, activity of NtmybA2 is also controlled at the post-translational level. We found that the transactivation potential of NtmybA2 is repressed by a regulatory domain located at its carboxyl terminus and that specific classes of cyclins A and B enhanced NtmybA2 activity possibly by relieving this inhibitory effect. Mutations at the 20 potential sites of phosphorylation by cyclin-dependent kinase (CDK) in NtmybA2 blocked the enhancing effects of the cyclins on NtmybA2 activity. Recombinant NtmybA2 was phosphorylated in vitro by a CDK fraction prepared from tobacco BY2 cells. The kinase activity for NtmybA2 in the CDK fraction was cell cycle-regulated in BY2 cells, peaking at the G(2)/M phase when the level of transcripts of cyclin B is maximal. Taken together, our data suggest that NtmybA2 is phosphorylated by a specific cyclin/CDK complex(es) at G(2)/M and that this phosphorylation removes the inhibitory effect of its C-terminal region, thereby activating NtmybA2 specifically at G(2)/M.
Araki
Satoshi
S
Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
Ito
Masaki
M
Soyano
Takashi
T
Nishihama
Ryuichi
R
Machida
Yasunori
Y
D001665:Binding Sites; D019925:Cyclin A / Q000378:metabolism; D019926:Cyclin B / Q000378:metabolism; D016213:Cyclins / Q000378:metabolism*; D018076:DNA, Complementary / Q000378:metabolism; D016195:G2 Phase; D017930:Genes, Reporter; D008938:Mitosis; D009154:Mutation; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary; D020598:Proto-Oncogene Proteins c-myb / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D013997:Time Factors; D014026:Nicotiana / Q000378:metabolism*; D014158:Transcription, Genetic; D015533:Transcriptional Activation; D014162:Transfection
D019925:Cyclin A; D019926:Cyclin B; D016213:Cyclins; D018076:DNA, Complementary; D020598:Proto-Oncogene Proteins c-myb; D011994:Recombinant Proteins