Regulation of a DNA-Compacting Plastid Nucleoid Protein
DCP68, a DNA-compacting nucleoid protein, was further characterized in order to understand how plastid nucleoid proteins affect the structure and function of chloroplast DNA. Previously, DCP68 was identified as ferredoxin:sulfite reductase, an enzyme that participates in reductive sulfur assimilation and inhibits chloroplast DNA replication and transcription in vitro [1,2]. In this study, the portion of SiR that was found to be present in soluble and plastid nucleoid-enriched fractions indicated that most SiR was stromal in Arabidopsis and soybean plants. Although SiR was detected in Arabidopsis chloroplast nucleoid-enriched fractions, the study of nucleoid dynamics proved to be difficult due to the few nucleoids that could be isolated from this model plant. Furthermore, Arabidopsis heterozygous mutants that contained a reduced SiR protein level did not display an obvious mutant phenotype that could be ascribed to the role of SiR in plastid nucleoids. A significantly higher amount of SiR was present in nucleoid-enriched fractions from young soybean leaves than in mature leaves. The variation in the amount of SiR allocated to plastid nucleoids supports the hypothesis that the interaction of SiR with ctDNA is regulated. The factors that may influence the association of SiR with plastid nucleoids remain elusive. In vitro evidence suggested that the phosphorylation status of SiR could potentially regulate its interaction with DNA . The isoelectric point profile of SiR was examined in vivo, as a first step towards identifying possible developmental differences in the post-translational modification of SiR. In conjunction with these studies, SiR was found to contain a conserved CK2 phosphorylation site and was capable of being phosphorylated by CK2 in vitro.