Transfer of rps14 from the mitochondrion to the nucleus in maize implied integration within a gene encoding the iran-sulphur subunit of succinate dehydrogenase and expression by alternative splicing

The maize mitochondrial genome does not contain a gene coding for ribosomal protein S14. In this paper we show that the functional rps14 gene was tran slocated to the nucleus and acquired the signals conferring expression and product targeting to the mitochondrion in a way not previously described. T ransferred rps14 was found integrated between both exons of a gene encoding the iron-sulphur subunit of the respiratory complex II (sdh2). Sdh2 exon 1 and rps14 were separated by a typical plant nuclear intron that was splice d to give a mature poly(A)+ mRNA of 1.4 kb. This processed mRNA encoded a c himeric SDH2 (truncated)-RPS14 polypeptide, and we show that this chimeric polypeptide is targeted into isolated plant mitochondria, where it is prote olytically processed in a complex way. An alternative splicing event utiliz ing the same 5' splice site and a different downstream 3' splice site gener ated a second mature poly(A)+ mRNA of 1.3 kb that contained both sdh2 exons . This sdh2 transcript encoded an SDH2 polypeptide highly conserved compare d with its homologues in other organisms, and it contained the three cystei ne-rich clusters that made up the three nonheme iron-sulphur centres respon sible for electron transport. To our knowledge, these results constitute th e first evidence of alternative splicing playing a role in the expression a nd targeting of two mitochondrial proteins with different functions from th e same gene.