The Arabidopsis locus RCB mediates upstream regulation of mitotic gene expression.

Transcriptional regulation of cell cycle regulatory genes, such as B-type cyclins, is tightly linked with the mitotic activity in the meristems. To study the regulation of a B-type cyclin gene, a targeted genetic approach was undertaken. An Arabidopsis line containing a fusion construct between the CYCB1;1 promoter and a bacterial beta-glucuronidase marker gene (uidA) was used in ethyl methanesulfonate mutagenesis. The mutants were screened for altered CYCB1;1::uidA expression patterns. In a reduced CYCB1;1 expression mutant (rcb), the CYCB1;1::uidA expression was severely affected, being excluded from the shoot and root apical meristems and leaf primordia and shifted to cells associated with root cap and stomata. In addition to the overall reduction of the endogenous CYCB1;1 transcript levels, other G2-to-M phase-specific genes were also down-regulated by the mutation. In the mutant plants, the inflorescence stem growth was reduced, indicating low meristem activity. Based on the altered CYCB1;1::uidA expression patterns in rcb root meristem, a model is proposed for RCB that mediates the tissue specificity of CYCB1;1 promoter activity.