Genes for the ribosomal S4 protein encoded in higher plant mitochondria are transcribed, edited, and translated

Here we describe the structure and expression of the mitochondrial gene that encodes the S4 subunit of the mitochondrial ribosome (rps4) in two higher plants, rapeseed and rice. The ORFs of rapeseed and rice rps4 genes are 362 and 352 codons long, respectively, and their corresponding transcripts are edited (C-to-U conversions) in 19 and 15 positions, respectively. The amino acid sequences deduced from the sequences of the two rps4 genes are 78.1% identical to each other and show about 50% identity to the liverwort (Marchantia polymorpha) mitochondrial RPS4 polypeptide. Both higher plant mitochondrial genes have an inserted sequence (of 137 and 141 codons for rapeseed and rice, respectively) relative to the liverwort rps4 gene. In rice rps4, the 3' portion of the inserted sequence can potentially form two stem-loop structures that resemble two domains characteristic of group II introns, i.e., domains V and VI. However, RT-PCR analysis showed that these sequences were not spliced out during maturation of the rps4 transcripts. Antibodies specific for the plant RPS4 proteins were prepared. Protein blot analysis revealed that RPS4 polypeptides are present in both rapeseed and rice mitochondria, whose sizes are in agreement with the molecular weights calculated from the non-spliced sequences. The rps4 genes and RPS4 polypeptides have been also identified in the mitochondria of wheat, maize, soybean, and turnip by Southern and Western analyses, respectively.