Nicotiana chloroplast genome : 6. Deletion and hot spot - a proposed origin of the inverted repeats.

A physical map containing six restriction sites of the Nicotiana tabacum chloroplast genome, together with the BamHI maps of N. tabacum, N. otophora and N. knightiana, and the SmaI maps of N. acuminata, N. plumbaginifolia, N. langsdorffii, N. otophora, N. tabacum, N. tomentosiformis and N. knightiana was constructed. In Nicotiana chloroplast genomes, the most frequently observed variations are point mutations. Deletions are also detected. Most of the observed changes are confined to one area of the large single copy region, which is designated as the "hot spot". Based on the evidence obtained from Nicotiana chloroplast genomes, an origin of the inverted repeats in this genus is proposed. We suggest that the inverted repeats represent a vestige of what were once two identical, complete chloroplast genomes joined together in a head-to-head and tail-to-tail fashion, and that deletions generated the current chloroplast genome organization.