Abstract
G2/M phase-specific gene transcription during plant cell cycle is mediated by R1R2R3-Myb transcription factors, which bind to mitosis-specific activator (MSA) elements. Arabidopsis thaliana contains five genes encoding for R1R2R3-Myb proteins, three of which (MYB3R1, MYB3R3 and MYB3R5) are acting as transcriptional repressor. Simultaneous mutations of the three repressor-type Myb genes resulted in significant increase of transcripts from G2/M phase-specific genes. We showed that such increase in expression of G2/M genes occurred predominantly in developmentally old tissues that have low or no cell division activity. In another experiment, we examined the expression pattern of CYCB1;1-GUS and CYCB1;2-GUS fusions in wild type and the triple mutant background. Wild type plants showed typical patchy pattern expression of GUS gene, whereas, in the triple mutant, GUS expression was uniformly increased while patchy pattern expression of GUS was unchanged. From these results, we concluded that repressor-type Myb may have roles for maintenance of repressed state of G2/M genes in quiescent cells, and for repressing transcription of G2/M genes during G1 and S phases in dividing cells.
