The gene for a subunit of an ABC-type heme transporter is transcribed together with the gene for subunit 6 of NADH dehydrogenase in rice mitochondria.

We previously identified a chloroplast-derived (ct-derived) sequence of 32 base pairs (bp) in rice mitochondrial DNA that includes a part (30 bp; psitrnI) of a gene for isoleucine tRNA (CAU) of the chloroplast. Analyzing the ct-derived psitrnI, we found that an open reading frame (orf240), which was homologous to the gene for a subunit of an ATP-binding cassette-type (ABC-type) heme transporter, namely helC, of Rhodobacter capsulatus, and a gene for subunit 6 of NADH dehydrogenase (nad6) were located upstream of and downstream from the ct-derived psitrnI, respectively. Northern-blot hybridization and analysis by reverse transcription-polymerase chain reaction (RT-PCR) revealed that both orf240 and nad6 were co-transcribed in rice mitochondria. An analysis of PCR-amplified fragments of the region of orf240/nad6 from the DNA of some Gramineae suggests that the arrangement of orf240/nad6 was generated in the mitochondrial genome of the genus Oryza during evolution after its divergence from the other Gramineae. Most of the transcripts of orf240 are edited, with a change from cytidine to uridine, at 35 positions. Editing of the RNA changes 33 amino-acid residues among the 240 encoded amino-acid residues, suggesting that the orf240 gene is functional in rice mitochondria.