G2/M-phase-specific transcription during the plant cell cycle is mediated by c-Myb-like transcription factors.
Plant B-type cyclin genes are expressed specifically in late G2- and M-phases during the cell cycle. Their promoters contain a common cis-acting element, called the MSA (M-specific activator) element, that is necessary and sufficient for periodic promoter activation. This motif also is present in the tobacco kinesin-like protein gene NACK1, which is expressed with timing similar to that of B-type cyclin genes. In this study, we show that G2/M-phase-specific activation of the NACK1 promoter also is regulated by the MSA element, suggesting that a defined set of G2/M-phase-specific genes are coregulated by an MSA-mediated mechanism. In a search for MSA binding factors by yeast one-hybrid screening, we identified three different Myb-like proteins that interact specifically with the MSA sequence. Unlike the majority of plant Myb-like proteins, these Myb proteins, NtmybA1, NtmybA2, and NtmybB, have three imperfect repeats in the DNA binding domain, as in animal c-Myb proteins. During the cell cycle, the level of NtmybB mRNA did not change significantly, whereas the levels of NtmybA1 and A2 mRNAs fluctuated and peaked at M-phase, when B-type cyclin genes were maximally induced. In transient expression assays, NtmybA1 and A2 activated the MSA-containing promoters, whereas NtmybB repressed them. Furthermore, expression of NtmybB repressed the transcriptional activation mediated by NtmybA2. Our data show that a group of plant Myb proteins that are structurally similar to animal c-Myb proteins have unexpected roles in G2/M-phase by modulating the expression of B-type cyclin genes and may regulate a suite of coexpressed genes.
Ito
M
M
Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Tokyo 113-0033, Japan. masakito@biol.s.u-tokyo.ac.jp
Araki
S
S
Matsunaga
S
S
Itoh
T
T
Nishihama
R
R
Machida
Y
Y
Doonan
J H
JH
Watanabe
A
A
D001483:Base Sequence; D018076:DNA, Complementary; D016195:G2 Phase; D017403:In Situ Hybridization; D008938:Mitosis; D008969:Molecular Sequence Data; D010947:Plants, Toxic; D011401:Promoter Regions, Genetic; D020598:Proto-Oncogene Proteins c-myb / Q000235:genetics / Q000502:physiology*; D014026:Nicotiana / Q000166:cytology*; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*; D014158:Transcription, Genetic / Q000502:physiology*
D018076:DNA, Complementary; D020598:Proto-Oncogene Proteins c-myb; D014157:Transcription Factors