TRANSFER OF RPS19 TO THE NUCLEUS INVOLVES THE GAIN OF AN RNP-BINDING MOTIF WHICH MAY FUNCTIONALLY REPLACE RPS13 IN ARABIDOPSIS MITOCHONDRIA

The discovery of disrupted rps19 genes in Arabidopsis mitochondria pro mpted speculation about the transfer to the nuclear compartment. We he re describe the functional gene transfer of rps19 into the nucleus of Arabidopsis. Molecular cloning and sequence analysis of rps19 show tha t the nuclear gene encodes a long N-terminal extension. Import studies of the precursor protein indicate that only a small part of this exte nsion is cleaved off during import. The larger part of the extension, which shows high similarity to conserved RNA-binding domains of the RN P-CS type, became part of the S19 protein. In the Escherichia coli rib osome S19 forms an RNA-binding complex as heterodimer with S13. By usi ng immune-analysis and import studies we show that a eubacterial-like S13 protein is absent from Arabidopsis mitochondria, and is not substi tuted by either a chloroplastic or a cytosolic homologue of this ribos omal protein. We therefore propose that either a highly diverged or mi ssing RPS13 has been functionally replaced by an RNP domain that most likely derived from a glycine-rich RNA-binding protein. These results represent the first case of a functional replacement of a ribosomal pr otein by a common RNA-binding domain and offer a new view on the flexi bility of biological systems in using well-adapted functional domains for different jobs.