Additional ORFs

ORFs encoding predicted polypeptides differing articulately from DNA and/or RNA polymerases exist in a number ofmitochondrial linear plasmids. Rather frequently, such reading frames overlap the DNA or RNA polymerase genes, as for pClK1, pClT5, and pAL-1 (Oeser and Tudzynski 1989; Oeser et al. 1993; Kempken et al. 1989).

In pMLP1 of Pleurotus ostreatus and pMC3-2 of Morchella conica large ORFs are located upstream of the DNA or RNA polymerase encoding genes (Rohe et al. 1991; Kim et al. 2000; Fig. 2). As the predicted protein of the respective pMLP 1 ORF is remarkably basic, it was presumed to function as a TIR binding protein, possibly involved in replication initiation (Kim et al. 2000). However, such information is not conserved among mitochondrial linear plasmids which rules out its general requirement. A function is not known or attributable to any of the known additional reading frames; there is no obvious similarity either to functionally assigned genes or predicted polypeptides, or to each other. Hence, mitochondrial linear plasmids generally encode exclusively TPs, DNA polymerase, and RNA polymerase, and seldomly nonessential other proteins.

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