A DNA-BINDING ACTIVITY FOR ONE OF 2 CLOSELY DEFINED PHYTOCHROME REGULATORY ELEMENTS IN AN LHCB PROMOTER IS MORE ABUNDANT IN ETIOLATED THAN IN GREEN PLANTS

The Lhcb2()1 gene of Lemna gibba is regulated positively by phytochro me, and two separate, 10-bp regions of this promoter have been shown t o be necessary for phytochrome regulation. We have now analyzed the ef fects of one and two base pair mutations to define exactly two cis ele ments within these regions that are necessary for phytochrome regulati on. These elements, designated RE alpha and RE beta, consist in part o f sequences highly conserved among promoters of genes encoding light-h arvesting chlorophyll alb proteins of photosystem II (Lhcb genes). The y are located -134 to -129 bp and -114 to -109 bp from the transcripti on start site, respectively. RE alpha has the sequence AACCAA and was found to interact specifically in vitro with a DNA binding activity in whole-cell extracts of plants. This activity was high in etiolated pl ants but much lower in green plants. RE beta has the sequence CGGATA. A GATA sequence created at a position six nucleotides upstream could r eplace the function of RE beta. We conclude that the phytochrome regul ation of Lhcb2()1 is mediated by at least two cis elements. These ele ments are likely to function by repression of the promoter activity in darkness, although the RE beta region also may be able to play a role in the activation of transcription.