Two E2F elements regulate the proliferating cell nuclear antigen promoter differently during leaf development

Area of Science:

• Plant molecular biology
• Cell cycle regulation
• Transcription factor function

Background:

• E2F transcription factors are crucial for cell cycle progression in eukaryotes.
• Plant E2F regulation of target genes, particularly at the G1/S boundary, remains poorly understood.
• Previous work identified two E2F binding sites (E2F1 and E2F2) in the Nicotiana benthamiana PCNA promoter, with E2F2 essential for repressing expression in mature leaves.

Purpose of the Study:

• To investigate the role of the distal E2F1 site and its interaction with the E2F2 site in regulating the PCNA promoter.
• To elucidate the differential contribution of E2F1 and E2F2 to PCNA gene expression in various plant tissues.

Main Methods:

• Gel shift assays using plant nuclear extracts and purified Arabidopsis E2F/DP proteins to analyze protein-DNA interactions.
• Site-directed mutagenesis of E2F1 and E2F2 binding sites within the PCNA promoter.
• Analysis of PCNA promoter activity in transgenic Nicotiana benthamiana plants.

Main Results:

• Distinct protein complexes bind to the E2F1 and E2F2 sites.
• Mutations in E2F1 and/or E2F2 sites differentially affected PCNA promoter activity in transgenic plants.
• Both E2F1 and E2F2 contribute to PCNA promoter repression in mature leaves; E2F1 mutation reduced activity in young leaves, while combined mutation had no effect, suggesting E2F1 counteracts E2F2 repression in young tissues.

Conclusions:

• The E2F1 and E2F2 elements play distinct roles in regulating the PCNA promoter.
• E2F1 and E2F2 cooperate to repress PCNA expression in mature leaves.
• In young tissues, E2F1 acts antagonistically to E2F2, modulating PCNA gene expression during the cell cycle.