Sigma factors are dissociable subunits of bacterial RNA polymerase that ensure efficient
transcription initiation from gene promoters. Owing to their prokaryotic origin, chloroplasts
possess a typical bacterial RNA polymerase together with its sigma factor subunit.
The higher plant Arabidopsis thaliana contain as many as six sigma factors for the
hundred or so of its chloroplast genes. The role of this relatively large number of
transcription initiation factors for the miniature chloroplast genome, however, is
not fully understood. Using two Arabidopsis T-DNA insertion mutants, we show that
sigma factor 1 (SIG1) initiates transcription of a specific subset of chloroplast
genes. We further show that the photosynthetic control of PSI reaction center gene
transcription requires complementary regulation of the nuclear SIG1 gene at the transcriptional
level. This SIG1 gene regulation is dependent on both a plastid redox signal and a
light signal transduced by the phytochrome photoreceptor.