Four Kinds of Plasmid-Iike DNA of Rice Mitochondria Are Transcribed

Abstract

In the mitochondria of rice (Oryza sativa L.), four kinds of small circular plasmid-like DNAs, designated B1, B2, B3 and B4, have been characterized. However, their function is unknown. To investigate their function, we analyzed the transcription of B1, B2, B3 and B4 by Northern blot hybridization and RNase protection assay. We found that B1, B2, B3 and B4 are transcribed in one direction in both seedlings and suspension-cultured cells of rice. From the results of RT-PCR and the Northern blot hybridization, it is suggested that B1, B2, B3 and B4, Iike other mitochondrial genes, are transcribed without termination and are processed to specific sizes of transcripts. These transcripts contain ORFs, but no homologous sequences were found in the databases except for an ORF in B3. Thus, it is still unclear whether these transcripts code functional proteins. Our results may complement the hypothesis concerning the transfer of plasmid-like DNA sequence via RNAs from mitochondria to nuclei.