Qft

At3g05740;

Hartung and Puchta 2006

Dual targeting or alternative translation start sites target product to both plastids and mitochondria. Length will vary depending on translation initiation at alternative start sites or at non-AUG start codons in the 5' UTR (Christensen et al. 2005). 2. Homologues of Oryza sativa (rice) plastid-localised proteins (Saotome et al. 2006). 3. Targeted to plastids and nuclei in O. sativa.

Dual targeting or alternative translation start sites target product to both plastids and mitochondria. Length will vary depending on translation initiation at alternative start sites or at non-AUG start codons in the 5' UTR (Christensen et al. 2005). 2. Homologues of Oryza sativa (rice) plastid-localised proteins (Saotome et al. 2006). 3. Targeted to plastids and nuclei in O. sativa.

possibly two catalytic subunits of 80 and 116 kDa for C. reinhardtii (Wang et al. 1991). The size discrepancies might be explained by proteolytic cleavage of proteins during purification. In the absence of primary sequence information the relationships between these proteins are not known and it is unclear whether they are different DNA polymerases or homologues of the same protein. Analyses of DNA polymerase genes present in sequenced genomes (discussed below; Mori et al. 2005) are likely to help resolve some of the discrepancies encountered in the earlier biochemical work. A number of proteins have been found to be associated with purified plastid DNA polymerases. These include a 43 kDa protein (related to ribonuclease T2, GenBank Acc. P93845) that stimulates the activity and proces-sivity of an 87 kDa (Chen et al. 1996) and a 70 kDa P. sativum DNA polymerase (Gaikwad et al. 2002), and a 3' to 5' 20 kDa exonuclease subunit of a 105 kDa S. oleracea DNA polymerase complex (Keim and Mosbaugh 1991).

There are similarities in the sizes and properties of DNA polymerases purified from plant mitochondria and chloroplasts (Heinhorst et al. 1990; Sakai et al. 1999). The idea of related DNA polymerases in mitochondria and plastids is supported by analyses of genes in sequenced genomes. The A. thaliana genome encodes at least two genes encoding organelle DNA polymerases (Table 2) sharing 70% amino acid identity that are expressed in the shoot apical meristem (Mori et al. 2005). The 116 kDa 1034-long A. thaliana DNA polymerase-like protein (gene no. At3g20540) contains a presequence that is predicted (Emanuelsson et al. 2000) to target mitochondria (TargetP score 0.74) and chloroplasts (TargetP score 0.59). A related 117 kDa, 1049 amino acid A. thaliana protein (gene no. At1g50840) contains a putative plastid targeting presequence (TargetP score 0.933). Both N-termini deliver GFP to chloroplasts but the 116 kDa presequence appears to also target GFP to the mitochondria (Mori et al. 2005). Interestingly, the 117 kDa protein is targeted to mitochondria when translation initiates upstream of the predicted AUG start codon most probably at an in frame CUG lying seven codons upstream of AUG (Christensen et al. 2005). Use of this non-AUG start codon suggests both A. thaliana organelle DNA polymerases are dual-targeted to plastids and mitochondria adding complexity to the regulation of organelle DNA poly-merases.

Homologues of the A. thaliana organelle targeted DNA polymerases are present in O. sativa. Two O. sativa organellar DNA polymerases are predicted to be dual-targeted to plastids and mitochondria (Christensen et al. 2005). Polyclonal antibodies raised against one of these polymerases cross-reacted with a protein in isolated chloroplasts (Kimura et al. 2002). Expression of this DNA polymerase was studied by in situ hybridization. RNA was detected in tissues with dividing cells including leaf primordia, and the apical meristem of shoots and roots but not in mature leaves (Kimura et al. 2002). These genome based studies suggest plastids contain at least two ~110 kDa DNA polymerases that appear to be dual-targeted to mitochondria and plastids (Christensen et al. 2005; Mori et al. 2005). Future experiments should delineate the roles of these polymerases in organelle DNA replication versus repair. Recombinant forms of these enzymes will facilitate the characterization of their properties (Mori et al. 2005). These A. thaliana and O. sativa organelle DNA polymerases contain 3'-5' exonuclease and DNA polA-like domains. The distantly related apicoplast in the malaria parasite Plasmodium falciparum appears to contain a different DNA polymerase. A 235 kDa multidomain protein with DNA primase, DNA helicase, DNA polymerase and 3' to 5' exonuclease regions is implicated in replication of apicoplast DNA (Seow et al. 2005).

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