Dispersed repeats and structural reorganization in subclover chloroplast DNA

The plastid genome from subclover, Trifolium subterraneum, is unusual in a variety of respects, compared with other land-plant chloroplast DNAs. Gene mapping of subclover chloroplast DNA reveals major structural reorganization of the genome. Ten clusters of genes are rearranged in both order and orientation. Eight large inversions are sufficient to explain this reorganization; however, the actual evolutionary changes may have been more complex. For example, a fine-scale analysis of a set of ribosomal protein genes reveals the occurrence of insertions, deletions, and transpositions. Associated with this unusually unstable genome are two structural features potentially involved in the rearrangements. A dispersed family of repeats, with each element about 1 kb in length, is present in at least six copies. A survey of a wide taxonomic range of species indicates that these elements are unique to the chloroplast DNAs of subclover and two closely related species. Several of the repeated elements are associated with genomic rearrangements, and one repeat is inserted within a normally highly conserved series of genes. This set of dispersed repeats may be the first family of transposable elements found in any organelle genome. In addition, the subclover genome is much larger than those in other closely related legumes, even when one takes into account the presence of the repeated elements. Some of the extra DNA has no sequence similarity to other chloroplast genomes and may represent insertion of DNA from another genome. These unusual features are not found in the structurally stable chloroplast genomes of other vascular plants and may, therefore, be implicated in the rapid and major reorganization of the chloroplast DNA in subclover.