Plastid DNA maintenance is governed by cis-acting plastid DNA sequences which are the targets for trans-acting proteins that replicate, recombine, and repair plastid genomes. The plastid genomes of green algae and plants that have been characterised do not encode any known DNA-RRR proteins. A number of non-green algae, including diatoms, red and cryptomonad algae contain a plastid dnaB-like gene (Kowallik et al. 1995; Reith and Munholland 1995; Douglas and Penny 1999). The dnaB gene encodes a DNA helicase involved in replication (Nakayama et al. 1984). In angiosperms the absence of plastid-encoded DNA-RRR proteins is demonstrated by the observation that plastid DNA is replicated in albino cereal (Hess et al. 1994; Zubko and Day 2002) and Brassica plants (Zubko and Day 1998) lacking plastid-encoded proteins.
DNA replication, recombination, and repair were once considered to be distinct pathways but more recent work in bacteria has shown they are interrelated processes (Kreuzer 2005). Pathways for recombination-dependent DNA replication and DNA replication-dependent recombination have been described (Kowalczykowski 2000; Kreuzer 2000, 2005) and are applicable to plastid DNA (see Section 7 below). This review will summarise our current knowledge on the mechanisms, DNA sequences and proteins involved in the maintenance of plastid DNA. We will focus on plastid DNA in flowering plants but will include relevant work from algal plastids where appropriate.
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