Abstract
We have obtained two cDNAs encoding bacterial DNA polymerase I homologs from BY-2 cultured tobacco cells, and named corresponding genes NtPolI-like1 and NtPolI-like2. The two genes exhibited 97-98% identity at nucleotide- and amino acid-levels. The NtPolI-like gene products exhibited DNA polymerase activity, were targeted to both plastids and mitochondria, and were localized in the plastid- and mitochondrial-nuclei. The apparent molecular masses, as well as biochemical properties of the gene products well agreed with those of DNA polymerases biochemically detected in the isolated organelle-nuclei. NtPolI-like transcript levels, NtPolI-like protein levels, and DNA polymerase activities in the organelle-nuclei concomitantly increase in the initial phase of cell proliferation, exactly when organelle DNA synthesis are transiently activated. The concomitant increase strongly suggest that NtPolI-like gene products are involved in the transient activation of organelle DNA synthesis in the initial phase of cell proliferation.
