~NA~TII~RE:....V:..:::O::L.~?JJI~2.!..!FE:::::B~RU:::.;AR:::.Y~I984~---------- NEWS AND VIEWS ___________________.:4.:.:15 Molecular biology function at some repeated elements - a possibility suggested by the finding of repeated elements in maize mitochondria DNA that are not surrounded by the Multiple forms of mitochondrial expected recombinant set of restriction fragments (D.M. Lonsdale, personal DNA in higher plants communication). Site-specific recombi­ from Thomas D. Fox nation would tend to limit the number of different circles that could be generated THE study of the structure of the mito­ species is to assume that they are related to from a master chromosome containing chondrial genomes of higher plants is a each other by homologous recombination many repeats. (The large number of linear daunting task. Simply in terms of size there between repeated elements. molecules in preparations of mito­ are problems enough - the genomes can As Palmer and Shields point out, this chondrial DNA from plants, as well as contain from about 200 kilobases (kb) to over relatively simple tripartite picture may turn yeast, are assumed by most workers to be 2,000 kb of primarily unique sequence, out upon further study to be complicated the result of in vitro degradation. Perhaps according to species. In comparison, by the presence of other minor species. One some of these linear molecules are human mitochrondrial DNA, which would, for example, expect the presence of recombination intermediates present in appears to code for roughly the same dimers of the 83 kb and 135 kb circles to be vivo.) number of products, is only 16.5 kb long. present. Furthermore, Palmer and Shields Progress in identifying plant mito­ Worse yet, electron microscopy of plant detected, but did not map, other minor chondrial genes through their homology mitochondrial DNA reveals a complex repeated sequences in Brassica mito­ with genes of other organisms will soon assortment of differently sized circular and chondrial DNA which may also serve as make it possible to see whether the arrange­ linear molecules, and restriction digests sites for recombination. ment of genes is conserved, as in animals, contain fragments in sub-stoichiometric Mapping studies on the much larger or not, as in fungi. Using the restriction amounts, suggesting a heterogeneous mitochondrial genome of maize by mapsofthemaizegenome, D.M. Lonsdale structure (for a review see ref. I). Lonsdale and co-workers at the Plant and C.J. Leaver of the University of Some semblance of order in this Breeding Institute and University of Edinburgh (personal communication) situation has now been provided by the Cambridge (personal communication) have already shown that the genes coding restriction mapping studies of Palmer and have indeed revealed a similar but more for cytochrome oxidase subunit I, apocyto­ Shields2, reported in this issue of Nature complex picture. They have mapped a chrome band a protein homologous to that (see p.437). Their results suggest that plant maize mitochondrial DNA master chromo­ encoded by the human unassigned reading mitochondrial genomes are carried on some of 570 kb and located six pairs of frame URFJ are present in a cluster. The discrete circular molecules which are repeated elements within it: five direct cluster also contains sequences derived derived from each other by recombination. repeats and one inverted repeat. In from chloroplast DNA containing what Chromosome walks along the mito­ addition, six other smaller circular maps, appear to be pseudogenes for chloroplast chondrial DNA of Brassica campestris ranging in size from 47 to 503 kb, were 16S rRNA 7 and the large subunit of 8 (Chinese cabbage, turnip) led around three generated and could be derived from the ribulose bisphosphate carboxylase • differently sized circles. The largest, master chromosome by recombination About 200 kb away is another gene, for 9 218 kb long, contains the entire sequence between pairs of repeated elements. These cytochrome oxidase subunit 11 , and complexity of the Brassica mitochondrial smaller circular maps seem to correspond midway between them, the rRNA genes. genome. This 'master chromosome' well in size to circles identified in maize Comparisons between species such as contains a major direct repeat of about mitochondrial DNA by electron micro­ Brassica and maize will also help give some 2 kb, whose elements are separated by scopy3. Thus the presence of multiple clues as to why the majority of plant mito­ 135 kb on one side and 83 kb on the other. circular species of DNA, related by chondrial DNA sequences appear not to The other two circles are 135 and 83 kb in recombination between repeated elements, code for useful products. That chloroplast size and each contains the corresponding may be a general feature of higher-plant DNA sequences 7 •8 are found in maize mito­ sequences present between the repeats of mitochondrial genomes. chondrial DNA led Lonsdale to suggest the master chromosome and a single copy The study of recombination in plant that the ability to take up foreign DNA of the repeated element. (Other arrange­ mitochondrial DNA has been hampered could in part account for the large and ments, such as dimers of the 218 kb circle, both by the lack of genetic markers and variable sizes of plant mitochondrial are consistent with the mapping data per se because mitochondrial DNA is maternally genomes. It will be interesting to see how but fail to explain the stoichiometries inherited. Nevertheless, evidence for true this is of other species, and whether observed for fragments containing the recombination has been obtained by bonafide nuclear DNA sequences also turn repeats and surrounding unique DNA.) somatic cell hybridization of strains up in plant mitochondria. D Clearly, a simple way to account for these differing in polymorphic mitochondrial DNA restriction fragments 4-6. The hybrids typically contain new combinations of Thomas D. Fox is at the Section of Genetics and years ago Development, Cornell University, Ithaca, New 100 parental fragment types, as might be York 14853. expected from the assortment of multiple BULLETIN No. 3 of the Entomological Division of the U.S. Department of Agriculture circular DNAs during mitotic growth. In addition, many hybrids contain novel I. Leaver, C.J. & Gray, M.W. A. Rev. Pl. Physiol. 33,373 (Washington, 1883), when stripped of the "red­ (1982). tape •• that appears to be even more necessary on fragments not found in either parent, 2. Palmer, J.D. & Shields, C.R. Nature 307,437 (1983). official documents in the States than it is in this which most probably arise as a result of 3. Levings, C.S. in Extrachromosomal DNA, /CN-UCLA country, is of more than usual interest. The Symposium on Molecular and Cellular Biology (eds true crossing-over. Complete restriction Cummings, D., Borst, P., Dawid, I., Weissman, S. & notorious "army-worm" appears in a new maps of plant mitochondrial DNAs will fox, C. f.) 63 (1979). character, viz, as destructive to cranberries, allow the detailed analysis of recombi­ 4. Belliard, G., Vedel, F. & Pelletier, G. Nature 281, 401 (1979). which form an important feature in the nation in somatic cell hybrids. production of the States. A long chapter is 5. Nagy, f., Torok, I. & Maliga, P. Mo/ec. gen. Genet. 183, Whether plant mitochondria contain a 437 (1981). devoted to the "cotton-worm" in which (in 6. Boeshore, M.L., Lifshitz, 1., Hanson, M.R. & Shamay, I. addition to interesting biological information) general recombination system, such as that Mo/ec. gen. Genet. 190, 459 (1983). elaborate contrivances for distributing arsenical present in yeast mitochondria, is not yet 7. Stern, D.B. & Lonsdale, D.M. Nature 229,698 (1982). 8. Lonsdale, D.M., Hodge, T.P., Howe, C.J. & Stern, D. B. solutions are described. clear. They may contain site-specific Ce/134, 1007 (1983). From Nature 29, 344, 7 February 1884. recombination systems which only 9. Fox, T.D. & Ledaver, C.J. Ce//26, 315 (1981). 0028~836/841050415~1SOI.OO C 1984 Macmillan Journals Ltd .