A nuclear sequence associated with self-incompatibility in Nicotiana alata has homology with mitochondrial DNA

A 1.0-kb nuclear fragment located 5' to a coding sequence associated with self-incompatibility in N. alata shows homology with mitochondrial chromosomal DNA on Southern blots. This sequence is also present in the mitochondrial DNA of two species of tomato, L. esculentum and L. pennellii, but shows no homology to mtDNA of Zea mays. The homologous mitochondrial fragment from N. alata was cloned and sequenced. A short region of 56 bp matches the nuclear sequence in 53/56 bp. Other matched but misaligned segments flank the 3' end. The nuclear sequence is marked at the 5' end by two 8 bp direct repeats. The function of the nuclear sequence is not known although, it is located 397 bp upstream from the site of transcription of the self-incompatibility gene. The mitochondrial sequence contains only limited open reading frames and the nuclear sequence has none. There is evidence that additional segments of the mitochondrial clone hybridize to other nuclear sequences. The exchange of sequences between the mitochondrial and nuclear genomes of plants is discussed.