Interaction of PvALF and VP1B3 domains with the β-phaseolin promoter

The phas promoter is potently transcribed during embryogenesis but in vegetative tissues it is completely silenced by a rotationally positioned nucleosome. Ectopic expression in leaves of PvALF, a seed-specific transcription factor belonging to the plant-exclusive B3 domain-containing VP1/ABI3 family, leads to chromatin remodeling of the phas promoter, permitting transcriptional activation by the growth regulator abscisic acid (ABA). Specific interaction with RY elements present in 40-42 bp oligonucleotide probes has been shown in vitro for Arabidopsis ABI3 and the isolated B3 domain of maize VP1. Here, both in vivo and in vitro approaches were used to show physical interaction of the B3 domain of VP1 or PvALF to RY elements in the native phas promoter. In electrophoretic mobility shift assays, small changes in B3 domain concentration differentiated between RY element-specific and sequence non-specific DNA binding. Increased affinity of the PvALF B3 domain to RY elements was observed in the presence of histories and other basic proteins, possibly reflecting the ability of this B3 factor to interact with the phas promoter in its nucleosomal configuration.