E2F sites that can interact with E2F proteins cloned from rice are required for meristematic tissue-specific expression of rice and tobacco proliferating cell nuclear antigen promoters

Plants have recently been found to have E2F-like and Rb-like proteins, regulators responsible for the G(1)(G(0))-S phase transition of the cell cycle in animals. Here we show that E2F is involved in transcription of plant genes for proliferating cell nuclear antigen (PCNA), which is required for DNA replication. Potential E2F binding sites found in the rice PCNA promoters mediated transcriptional activation in actively dividing cells and tissues of tobacco, but not transcriptional repression in terminally differentiated tissues, as also observed for the PCF binding sites previously found in the rice promoter. Similar results were obtained from analyses for a PCNA promoter isolated from tobacco, which contained two E2F-like sites, each with a different degree of contribution to the promoter activation. These E2F-like sites except for a rice site were indeed bound specifically by recombinant proteins of rice E2F, OsE2F1 and OsE2F2, and complexes of OsE2F1 with Arabidopsis DP proteins. Furthermore, OsE2F1 had the ability to transactivate an E2F-reporter gene containing the tobacco E2F site on co-expression with an Arabidopsis DP, and the transactivation was greatly enhanced by tagging a canonical nuclear localization signal to OsE2F1, suggesting a nuclear import-mediated regulation of the OsE2F1 function. In addition, we found that a large number of replication- and mismatch repair-associated genes in Arabidopsis contain E2F binding sequences conserved in their predicted promoter regions.