UNIQUE FEATURES OF THE MITOCHONDRIAL ROLLING CIRCLE-PLASMID MP1 FROM THE HIGHER-PLANT CHENOPODIUM-ALBUM (L.)

We analyzed the structure and replication of the mitochondrial (mt) ci rcular DNA plasmid mp1 (1309 bp) from the higher plant Chenopodium alb um (L.). Two dimensional gel electrophoresis (2DE) revealed the existe nce of oligomers of up to a decamer in addition to the prevailing mono meric form. The migration behavior of cut replication intermediates du ring 2DE was consistent with a rolling circle (RC) type of replication . We detected entirely single-stranded (ss) plasmid copies hybridizing only with one of the two DMA strands. This result indicates the occur ence of an asymmetric RC replication mechanism, mp1 has, with respect to its replication, some unique features compared with bacterial RC pl asmids. We identified and localized a strand-specific nicking site (or igin of RC replication) on the plasmid by primer extension studies. Ni cks in the plasmid were found to occur at any one of six nucleotides ( TAAG/GG) around position 735 of the leading strand. This sequence show s no homology to origin motifs from known bacterial RC replicons. mp1 is the first described RC plasmid in a higher plant.