EXPANSION OF THE IR IN THE CHLOROPLAST GENOMES OF BUCKWHEAT SPECIES IS DUE TO INCORPORATION OF AN SSC SEQUENCE THAT COULD BE MEDIATED BY ANINVERSION

The chloroplast genomes in buckwheat species contain large inverted re peats which are at least 4 kbp longer than the majority of those in la nd plants. The length of the buckwheat inverted repeats was attributab le to an additional region located adjacent to the borders of the smal l single-copy region. We have cloned and sequenced a 5.2-kbp SmaI frag ment corresponding to this extra region in the inverted repeats. A hom ology search revealed that the sequence of the SmaI fragment is highly homologous to one side of the small single-copy region of the inverte d repeats in dicot chloroplast DNAs such as tobacco and beechdrops. In terestingly, a 3.7-kbp segment in the middle of the SmaI fragment is i nserted in the opposite orientation relative to those of the other dic ot species, and 17-bp direct repeats are found located at both the end s of the additional region. These results suggest that expansion of th e inverted repeats in buckwheat chloroplast DNA might have been associ ated with an inversion.