THE RPS4-GENE IS ENCODED UPSTREAM OF THE NAD2-GENE IN ARABIDOPSIS-MITOCHONDRIA

In Arabidopsis mitochondria the nad2-gene consists of five exons (a-e) which are separated by three cis-splicing introns and one trans-splic ing intron. Sequence analysis of the region upstream of exons a and b reveals an open reading frame encoding ribosomal protein S4 (rps4). In the second nad2 coding region (exons c-e) a pseudo tRNA(Tyr) sequence and a fragment of the plastid psbA gene are located upstream of the t rans-spliced exon c. Primer extension analysis identifies RNA 5'-termi ni within the pseudo-tRNA(Tyr) confirming this sequence to be non-func tional. Northern blot analysis suggests the rps4-gene to be cotranscri bed with at least the first part of the nad2-gene. The rps4 and nad2 c oding sequences as well as the first cis-intron and the trans-intron s equences of the nad2 gene are altered by RNA editing. RNA editing in t he open reading frames improves in most instances conservation of the specified amino acids.