Cell-cycle, phase-specific activation of Maize streak virus promoters

It is believed that geminiviral DNA replication is coupled to the cell-cycl e regulatory complex of the plant cell and that the virus-early (complement ary or C sense) gene products REP and REPA may be able to manipulate the re gulation of the cycle. In this study, we examined expression from the promo ters of Maize streak virus (MSV) in transgenic maize plants and cells to de termine whether they showed cell-cycle specificity. Histochemical staining of plant roots containing "long and short" C-sense promoter sequences upstr eam of the GUS (beta -glucuronidase) reporter gene showed that promoter act ivity was restricted to the meristematic region of the roots and was enhanc ed by 2,4-dichlorophenoxy acetic acid (2,4-D) treatment. Analysis of report er gene and cell-cycle-specific gene transcript levels coupled with flow cy tometric data in synchronized transgenic maize cells revealed that all of t he MSV promoters showed cell-cycle specificity. The coat protein gene promo ter showed highest activity in early G2, whereas the C-sense promoter seque nces produced two peaks of activity in the S and G2 cell-cycle phases.