Abstract
Plastid DNA forms a nucleoid with a number of proteins. In pea and soybean, sulfite reductase (SiR) was identified as a main constituent in isolated plastid nucleoids, and was reported to repress transcription in vitro. SiR catalyzes reduction of sulfite to sulfide using ferredoxin as an electron donor and has been believed to exist in plastid stroma. Immunofluorescent staining of isolated pea chloroplasts with anti-SiR antibody showed localization of SiR to nucleoids. DNA-binding properties of SiR were analyzed by gel mobility shift assay using recombinant pea SiR. Stepwise band shift was observed, suggesting that different number of SiR molecules were bound. Apparent dissociation constants for 40mer and 20mer double-stranded DNAs were ~25 nM and ~225 nM, respectively. Single-stranded DNA was also shifted. Available evidence suggests that the DNA-binding was not sequence-specific. These results suggest that SiR binds to entire plastid genome, involved in the transcriptional repression.
