Analysis of nuclear sequences homologous to the B4 plasmid-like DNA of rice mitochondria; evidence for sequence transfer from mitochondria to nuclei.

Nuclear sequences homologous to the plasmid-like DNA, B4, were analyzed in the Japonica rice variety, Fujiminori. Homologous sequences existed at several positions in the nuclear genome, but each contained only a portion of the B4 sequence. It was impossible to reconstruct the entire sequence of B4 even by collating all the homologous sequences. Overlaps between some of the B4 sequences present in the nuclear genome resulted in parts of the sequence being represented more than once. These features indicate that nuclear sequences homologous to B4 are not the origin of B4 and that they have been transferred from mitochondria and integrated into the nuclear genome. Five other foreign sequences originating in the chloroplast or mitochondrial genome were found within 1 kb of the B4-homologous sequences. Structural analysis is consistent with the hypothesis that the DNA sequences were transferred via RNA.