A DNA binding activity for one of two closely defined phytochrome regulatory elements in an Lhcb promoter is more abundant in etiolated than in green plants.

The Lhcb21 gene of Lemna gibba is regulated positively by phytochrome, and two separate, 10-bp regions of this promoter have been shown to be necessary for phytochrome regulation. We have now analyzed the effects of one and two base pair mutations to define exactly two cis elements within these regions that are necessary for phytochrome regulation. These elements, designated REalpha and REbeta, consist in part of sequences highly conserved among promoter of genes encoding light-harvesting chlorophyll a/b proteins of photosystem II (Lhcb genes). They are located -134 to -129 bp and -114 to -109 bp from the transcription start site, respectively. REalpha 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 plants but much lower in green plants. REbeta has the sequence CGGATA. A GATA sequence created at a position six nucleotides upstream could replace the function of REbeta. We conclude that the phytochrome regulation of Lhcb21 is mediated by at least two cis elements. These elements are likely to function by repression of the promoter activity in darkness, although the REbeta region also may be able to play a role in the activation of transcription.