Packaging of plastid DNA

Within plastids, the DNA is not dispersed but localised into aggregates of DNA and protein called nucleoids (Kuroiwa 1991; Sakai et al. 2004). The uniformity of plastid DNA is governed by DNA-RRR pathways that are likely to be carried out in nucleoids. The organisation of multiple plastid genome copies into a smaller number of units will govern the segregation of plastid DNA during plastid and cell divisions (VanWinkle-Swift 1980) and will facilitate cytoplasmic sorting. The number, sizes, morphologies, and distribution of nucleoids, visualised by DAPI staining, vary during development of chloroplasts from proplastids (Miyamura et al. 1986). T. aestivum proplastids contain one to ten nucleoids and 30 to 40 plastid genomes whereas chloroplasts contain ten to thirty nucleoids and 70 to 100 plastid genomes (Miyamura et al. 1990). In Nicotiana, mature chloroplasts contain eight to forty nucleoids, each with about ten plastid genomes (Kuroiwa 1991). Nu-cleoids appear to be located in the stroma or attached to the envelope or thylakoids depending on the plastid type (Sato et al. 2003; Sakai et al. 2004). The functional significance of changes in the intra-plastidic location of nucleoids is not known. However, it is interesting to note that constitutive expression of the B. napus homologue of the P. sativum plastid envelope DNA (PEND) binding protein in N. tabacum nuclear transformants leads to an albino phenotype possibly due to a lack of release of DNA from the envelope (Wycliffe et al. 2005). The PEND protein is targeted to plastids and might be involved in anchoring plastid DNA to the inner envelope during early chloroplast development (Sakai et al. 2004),

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