Rice mitochondrial genome contains a rearranged chloroplast gene cluster

We have previously reported the isolation and partial sequence analysis of a rice mitochondrial DNA fragment (6.9 kb) which contains a transferred copy of a chloroplast gene cluster coding for the large subunit of ribulose-1,5-bisphosphate carboxylase (rbcL), β and ε subunits of ATPase (atpB and atpE), methionine tRNA (trnM) and valine tRNA (trnV). We have now completely sequenced this 6.9 kb fragment and found it to also contain a sequence homologous to the chloroplast gene coding for the ribosomal protein L2 (rpl 2), beginning at a site 430 bp downstream from the termination codon of rbcL. In the chloroplast genome, two copies of rpl 2 are located at distances of 20 kb and 40 kb, respectively, from rbcL. We have sequenced these two copies of rice chloroplast rpl 2 and found their sequences to be identical. In addition, a 151 bp sequence located upstream of the chloroplast rpl 2 coding region is also found in the 3′ noncoding region of chloroplast rbcL and other as yet undefined locations in the rice chloroplast genome. Hybridization analysis revealed that this 151 bp repeat sequence identified in rice is also present in several copies in 11 other plant species we have examined. Findings from these studies suggest that the translocation of rpl 2 to the rbcL gene cluster found in the rice mitochondrial genome might have occurred through homologous recombination between the 151 bp repeat sequence present in both rpl 2 and rbcL.