Impaired splicing of the rps12 transcript in ribosome-deficient plastids.

Analysis of RNA maturation in ribosome-deficient plastids of four non-allelic barley mutants revealed an increased accumulation and altered processing of transcripts of the ribosomal protein gene CS12 (rps12) compared to normal chloroplasts. The three exons of rps12 are part of two different polycistronic transcription units. Generation of mature rps12-mRNA involves both cis- and trans-splicing. In ribosome-deficient plastids, the cis-intron separating exons 2 and 3 remains entirely unspliced whereas the splicing of the bipartite rps12 trans-intron between exon 1 and exon 2 occurs, but at a reduced level. A comparison of the 3' and 5' ends of the two RNAs that are generally assumed to interact during trans-splicing showed a difference in the processing pathways of 3' rps12 transcripts between mutated and normal chloroplasts. Nonetheless, the final products were identical.