Plant mitochondrial PORR proteins facilitate group II intron splicing by binding to distinct regions within their target introns.

Ribozymic introns are widely distributed in eubacteria and organelles such as mitochondria and chloroplasts. In plants, organellar introns often exhibit degenerate RNA structures, lacking essential elements for self-splicing and mobility. Consequently, their splicing relies heavily on host-encoded proteins. The plant organellar RNA recognition (PORR) domain, a recently identified RNA-binding motif, defines a small, plant-specific protein family. In this study, we characterized five novel mitochondria-targeted PORR genes. Functional analyses revealed that all five PORRs play a role in mitochondrial group II intron splicing. While one PORR protein was found necessary for the splicing of a single intron, the other four facilitate the splicing of multiple introns. Splicing defects in each porr mutant resulted in a significant reduction in complex I assembly and activity, along with an increase in the levels of other respiratory complexes. In vivo RNA binding studies revealed that the analyzed PORR proteins bind either to the 3' end of trans-spliced intron 5'-halves or within intron domain I, indicating potential roles in trans-spliced intron rejoining or in the structural organization of domain I. Overall, these findings demonstrate that nuclear-encoded PORR proteins play crucial roles in the splicing of specific mitochondrial group II introns and are essential for mitochondrial biogenesis.