Two collections of Arabidopsis GAL4 enhancer trap lines were screened for
light-intensity dependent reporter gene activation. Line N9313 was isolated
for its strong light-intensity regulation. The T-DNA element trapped distant
enhancers of the SIG5 promoter, which drives expression of a sigma factor
involved in regulation of chloroplast genes for photosystem II core proteins.
The T-DNA insertion 715 bp upstream of the transcription initiation site
splits the promoter in a distal and proximal part. Both parts are sensitive to
blue and red light and depend on photosynthetic electron transport activity
between photosystem II and the plastoquinone pool. The mainblue-light
sensitivity is localized within a 196-bp sequence (–887 to –691 bp) in the
proximal promoter region It is preferentially CRY1 and PHYB controlled. Type-I
and type-II phytochromes mediate red-light sensitivity via various promoter
elements spread over the proximal and distal upstream region. This work
characterizes SIG5 as an anterograde control factor of chloroplast gene
expression, which is controlled by chloroplast signals in a retrograde manner.
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