Mitochondrial DNA displacement loop structure depends on growth state in bovine cells

One portion of the mitochondrial genome of mammals contains no structural genes. This so-called displacement loop (D-loop) region includes the two major transcriptional promoters and one of the origins of DNA replication. The D-loop region is often found in a novel triplex structure in which a discrete single-stranded DNA segment is hybridized to one strand of the genome displacing the other strand. The significance of this triplex structure and the possible function of the single-stranded D-loop segment have remained obscure. Here we show that the locations of the major 5' termini of D-loops in growing and nongrowing bovine cells differ by 46 nucleotides. The location of bovine D-loop 5' termini is dependent on the cell cycle but is not directly linked to nuclear DNA replication. The nature of D-loop 5' termini in growing cells is altered by inhibitors of cytoplasmic protein synthesis but not by inhibitors of mitochondrial protein synthesis. Changes in D-loop 5' termini correlate with alterations in transcripts from one of the transcriptional promoters. These findings implicate D-loops in the control of bovine mitochondrial gene expression and suggest mechanisms by which this control might by accomplished.