Abstract
Plant sulfite reductase (SiR) reduces sulfite to sulfide by accepting electrons from ferredoxin in plastids. SiR has been found in isolated chloroplast nucleoids. We examined the DNA-binding properties of SiRs from pea (PsSiR) and maize (ZmSiR) using an enzymatically active holoenzyme. PsSiR bound to both double-stranded and single-stranded DNA without significant sequence specificity. DNA binding did not affect the enzymatic activity, suggesting that ferredoxin and sulfite are accessible to SiR within the nucleoids. The DNA-compacting ability is higher in PsSiR than in ZmSiR. The tight compaction of nucleoids by PsSiR led to severe repression of transcription activity in nucleoids. Immunofluorescence microscopy showed that the majority of SiR co-localized with nucleoids in pea chloroplasts, whereas no particular localization to nucleoids was detected in maize chloroplasts. These results suggest that SiR plays an essential role in compacting nucleoids, but that the extent of association of SiR with nucleoids varies among plant species.
