Abstract
Transcription factor IIIA (TFIIIA) is required for eukaryotic synthesis of 5S ribosomal RNA by RNA polymerase III. Here we report the discovery of a structured RNA element with clear resemblance to 5S rRNA that is conserved within TFIIIA precursor mRNAs from diverse plant lineages. TFIIIA protein expression is controlled by alternative splicing of the exon containing the plant 5S rRNA mimic (P5SM). P5SM triggers exon skipping upon binding of ribosomal protein L5, a natural partner of 5S rRNA, which demonstrates the functional adaptation of its structural mimicry. As the exon-skipped splice product encodes full-length TFIIIA protein, these results reveal a ribosomal protein-mRNA interaction that is involved in 5S rRNA synthesis and has implications for cross-coordination of ribosomal components. This study also provides insight into the origin and function of a newfound class of structured RNA that regulates alternative splicing.