Abstract
We have isolated two genes encoding bacterial DNA polymerase I homologues from BY-2 cultured tobacco cells, and named them NtpolI-like 1 and NtpolI-like 2. The gene products were 122 (mature) to 128 kDa (precursor) in deduced sizes, 97.2% identical at amino-acid sequence level with each other, and predicted to be targeted to plastids. Recombinant NtpolI-like2 protein actually exhibited DNA polymerase activity in vitro, and its characters coincided with those of DNA polymerase(s) present in the plastid-nuclei (nucleoids) isolated from BY-2 cells. These results strongly suggest that NtpolI-like1/2 gene products are plastid DNA polymerases. However, western blotting analyses using antibody raised against NtpolI-like2 protein demonstrated the presence of approximately 120-kDa, NtpolI-like2-related proteins in both plastid- and mitochondrial-nuclei, suggesting that (1) either or both of the NtpolI-like1/2 gene products are dually targeted to plastids and mitochondria, or (2) mitochondrial DNA polymerase is distinct from, but are quite similar to, NtpolI-like1/2 gene products.
