We isolated and characterized two rice genes, OsRpoTp and OsRpoTm, that encode putative phage-type RNA polymerases. Predicted amino acid sequences showed high homology of these genes to known RpoT genes. A transient expression assay using green fluorescent protein indicated that the encoded proteins were localized to plastids and mitochondria, respectively. We demonstrated by reverse transcription-PCR experiments and immunoblot analysis that OsRpoTp expression occurred at an early stage of leaf development, prior to the transcript accumulation of the genes that were transcribed by the nuclear-encoded plastid RNA polymerase (NEP). Expression analyses of the chloroplast-deficient rice mutant, virescent-1, showed a discrepancy between OsRpoTp protein accumulation and the level of transcripts of NEP-transcribed genes. Our results suggest that NEP activation is regulated by a process after transcription, and is affected by the developmental state of chloroplast biogenesis.
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