Phage T4-like intermediates of DNA replication and recombination in the mitochondria of the higher plant Chenopodium album (L.)

We have studied intermediates of the recombination and replication of chrom osomal mitochondrial (mt) DNA prepared from suspension cultured cells of Ch enopodium album (L.) by electron microscopy during the whole growth cycle. We identified several types of potential recombination and replication inte rmediates including rosette-like structures, as well as other branched and sigma-like molecules. The absolute and relative amounts of these structures changed dramatically during the growth cycle, indicating high dynamics in the structural organization of the mt genome. The rosette-like molecules ha d sizes of 2-5 genome units and were found to contain putative replication forks and 'Holliday'-junctions known from recombination intermediates. The high number of rosettes during the first days of culture, and their drastic reduction in the stationary growth stage, were found to be inversely relat ed to changes in the quantity of linear molecules of 40-200 kb. This observ ation suggests that linear molecules participate in the formation of giant branched rosette-like structures. Most linear molecules were previously fou nd to have at least one single-stranded end, which may allow recombinative invasion of other double-stranded molecules. Thus, recombination events may lead to the formation of more complex molecules and initiate replication s imilar to phage T4. We propose the coexistence of a recombination-dependent mode of replication with a presumably recombination-independent rolling-ci rcle mode of replication in the mitochondria of C. album.