The gene for mitochondrial ribosomal protein S14 has been transferred to the nucleus in Arabidopsis thaliana

The transfer of genetic information from the mitochondrion to the nucleus i s thought to be still underway in higher plants. The mitochondrial genome o f Arabidopsis thaliana contains only one rps14 pseudogene. In this paper we show that the functional gene encoding mitochondrial ribosomal protein S14 has been translocated to the nucleus. This gene transfer is a recent evolu tionary event, which occurred within Cruciferae, probably after the diverge nce of Arabidopsis and Brassica napus. A 5' extension of the rps14 reading frame encodes a presequence which, in vitro, targets the polypeptide to iso lated mitochondria and is cleaved off during or after import. No intron was found at the junction of the targeting presequence with the mitochondriall y derived sequence, which are directly connected. By contrast, a 90-bp intr on, which is removed by splicing to give a mature poly(A)(+) mRNA of 0.9 kb , is located in the 3' non-coding region. To our knowledge, this is the fir st report of an intron in such a position in a functional transferred gene in higher plants, and suggests that exon shuffling may have been involved i n the acquisition of elements necessary for expression in the nucleus. Puta tive roles of this intron in polyadenylation and enhancement of gene expres sion are discussed.