We have characterized a DNA-binding activity, designated light-regulated nuclear factor (LRF-1), which interacted with a specific sequence located 150 nucleotides upstream from the transcription start site of a phytochrome-regulated Lemna gibba rbcS gene (SSU5B). There was a higher level of LRF-1 activity recovered from nuclei of light-grown plants than from dark-treated plants. In light-grown plants given a 1-day dark treatment, either white light or a single 2-min red illumination caused a rapid twofold to threefold increase in this activity, suggesting that the phytochrome system is probably involved in its regulation. The nuclear extracts also contained an activity that bound specifically to Box II sequences from a pea rbcS gene [Green, P.J., Yong, M.H., Cuozzo, M., Kano-Murakami, Y., Silverstein, P., and Chua, N.-H. (1988). EMBO J. 7, 4035-4044], but this activity was not higher in the light-grown compared with the dark-treated plants. Comparison of about 700 base pairs upstream from the SSU5B transcription start site with the upstream sequences of two other Lemna rbcS genes revealed several conserved regions. One of these regions is found upstream of rbcS genes in other species and is contained in the sequence which was shown to interact with LRF-1.
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