Chloroplast DNA sequences integrated into an intron of a tomato nuclear gene

DNA sequences capable of hybridizing with chloroplast DNA have previously been reported to exist in the nuclear genome of higher plants. Here we show that the third intron of the cultivated tomato (Lycopersicon esculentum) nuclear gene Cab-7, which resides on chromosome 10 and which we recently cloned and sequenced, contains two DNA fragments derived from the coding region of the chloroplast gene psbG. The first fragment, 133 bp long, is located at a site 63 bp from the 3' end of the 833 bp intron. The exact sequence of the 11 nucleotides at the 3' end of the inserting chloroplast sequence is also found at the 5' border of the insertion. A small (107 bp) chloroplast DNA fragment is inserted near the middle of the intron, again with the 3' end of the inserting element (6 bp) duplicated at the 5' border of the insertion. The second insert is a subfragment of the first insert, and is most likely directly derived from it. The psbG insertion sequence was found to be present in the Cab-7 gene of all tomato species examined but not in species from related genera (e.g. Solanum, Petunia, Nicotiana), suggesting that the original transposition event (chloroplast to nucleus) occurred relatively recently--since the divergence of the genus Lycopersicon from other genera in the family Solanaceae, but before radiation of species in that genus.