From a BamHI library of cms-T mitochondrial DNA, a 5.5 kbp cloned fragment, designated as T7, was isolated and shown to contain sequence homology to chloroplast DNA. Chloroplast homology was shown to be within a 1.95 kbp EcoRI to BamHI fragment of the T7 clone, and the nucleotide sequence of this fragment was determined. Subclones for DNA sequencing were generated by unidirectional exonuclease III digestion of the 1.95 kbp fragment (Henikoff, 1984, Gene 28:351). Homology to the 3'-end of the 23S rRNA, all of the 4.5S and 5S rRNAs, and to a nearly complete copy of the tRNAArg gene was found on a 1270 bp contiguous stretch of DNA. Bionet computer programs were used to identify and compare regions of homology. Where the maize chloroplast sequence has been determined, the DNA sequence homology was greater than 90%; comparisons with other chloroplast sequences from tobacco and Spirodela showed approximately 85% homology.
In the chloroplast genome the 23S-4.5S-5St-RNAArg cluster of genes is located with the inverted repeats. In the T7 clone, the gene order is the same as in the chloroplast DNA. Moreover, the spacing and degree of sequence homology of the small intergenic regions is similar between the chloroplast DNA and the inserted DNA in clone T7. The organizational conservation of the chloroplast genes and their intergenic spaces suggested that the transfer of this DNA occurred as a single event. Interestingly, portions of the T7 clone are duplicated elsewhere in the mtDNA of T cytoplasm. Partial homology to the T7 clone is found in a 3.1 kbp BamHI fragment. Additionally, the partial tRNAArg sequence has been found within a 9.0 kbp BamHI fragment (Dewey et al., 1986, Cell, in press).
Previous reports of homologous sequences between the chloroplast and mitochondrial genomes suggest that several interorganelle exchanges of DNA may have occurred (Stern and Lonsdale, 1982, Nature 299:698; Lonsdale et al., 1983, Cell 34:1007; Stern and Palmer, 1984, PNAS 81:1946). DNA sequence comparisons at the junctions of chloroplast DNA insertions may help elucidate the mechanism(s) by which these organelle exchanges occur.
C. J. Braun and C. S. Levings III
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