R-plasmid homology regions in mtDNA of the RU maize line possessing low copies of R-plasmids
--E. V. Kuzmin and N. V. Lavrova

RU cytoplasm found in several Latin American indigenous maize lines (D. H. Timothy et al., Maydica 28:139-149, 1983) can be distinguished by the presence of two mitochondrial linear plasmids--R1, 7.4kbp and R2, 5.4kbp--that are thought to be ancestors of S-plasmids. There is no extensive published data so far concerning the organization of integrated copies of R-plasmids present in the mitochondrial chromosome of RU lines. In the course of screening of different local maize lines from Latin America we have found a Bolivian line possessing free R-plasmids present in a comparable copy number to that of ubiquitous N-plasmid. This cytoplasm differs from Enano RU cytoplasm where both R-plasmids are present in several times higher copy numbers than N-plasmid.

Chromosomal mtDNA of the above-mentioned Bolivian line (Inst. of Plant Industry intermediate cat. No. 3888) was electrophoretically purified from free R-plasmids and blot-hybridizations were performed with the following probes: terminal inverted repeat (IR) of R-plasmids; ORF3 of S1; ORF1 of S2; ORF2 of S1/S2; 5kb-repeat of N-type mtDNA. The results of this analysis showed that this type of RU cytoplasm is very similar to N. Its mitochondrial chromosome contains only one incomplete integrated copy of each R-plasmid adjacent to the 5kb-repeat. The organization of the R2-homology region is indistinguishable from N-type mtDNA, but R1 is integrated in the opposite direction relative to the 5kb-repeat. Both R1 and R2 integrated copies are comparable in size with free R1 and R2 but possess only one IR. No complete integrated R-plasmids with one free linear end similar to S-plasmid integrates in S cytoplasm were found. It could be due to low R-plasmid copy number and resultant low effectivity of homologous recombination between free plasmid IR and IR-part of 5kb-repeat.

If only integrated copies of R-plasmids can be transcribed then line 3888 would possess ORF1 and ORF3 but no ORF2 transcripts in its mitochondria, which is opposite to the situation in other RU lines where ORF2 transcript can be detected readily (P. L.Traynor and C. S. Levings, III,  Plant Mol. Biol. 7:255-263, 1986). This fact could be one of the causes of low R-plasmid copy number. Another cause--the influence of particular nuclear background--cannot be ruled out. The experimental proof of this prediction is now going on.

The authors are grateful to the Maize Division of the Institute of Plant Industry in St. Petersburg for the generous gift of seeds. 

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