Studies on bacteria have identified a suite of DNA metabolism enzymes including DNA polymerases, DNA primase, RecA, topoisomerases, and helicases (CameriniOtero and Hsieh 1995). The cyanobacterium Anabaena contains 93 genes encoding proteins with significant similarity to known DNA-RRR proteins (Kaneko et al. 2001). Early studies to identify plastid DNA-RRR proteins involved purifying enzymes with DNA-RRR activities, such as DNA synthesis, from chloroplasts (McKown and Tewari 1984). More recently, whole genome da tabases have been used to identify candidate plastid-targeted proteins with significant matches to well-known bacterial DNA-RRR proteins. Further experimental support is then required to confirm in silico predictions of plastid location. The list of genes encoding homologues of DNA-RRR proteins for which there is experimental support for a plastid location is short and is reviewed below. Proteomics of purified chloroplasts (Chapter 12) provides an alternative approach to identify DNA-RRR proteins. However, the limited abundance of plastid DNA-RRR proteins hinders their identification in whole chloroplast preparations. Further purification of sub-chloroplast fractions containing DNA-protein complexes is required to identify plastid DNA-RRR proteins (Sakai et al. 1999; Phinney and Thelen 2005).
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