An organellar maturase associates with multiple group II introns

Abstract

Bacterial group II introns encode maturase proteins required for splicing. In organelles of photosynthetic land plants, most of the group II introns have lost the reading frames for maturases. Here, we show that the plastidial maturase MatK not only interacts with its encoding intron within trnK-UUU, but also with six additional group II introns, all belonging to intron subclass IIA. Mapping analyses of RNA binding sites revealed MatK to recognize multiple regions within the trnK intron. Organellar group II introns are considered to be the ancestors of nuclear spliceosomal introns. That MatK associates with multiple intron ligands makes it an attractive model for an early trans-acting nuclear splicing activity.roup II introns are highly structured ribozymes, found in bacteria and organellar genomes of plants, fungi, protists, and some animals. They catalyze their own excision from precursor RNAs (1). In addition, bacterial group II introns are mobile genetic elements. These features, as well as structural similarities with nuclear introns, have led to the proposition that ancestors of modern group II introns have crossed kingdoms from prokaryotes to eukaryotes via eubacterial endosymbionts, and eventually evolved into nuclear introns (2). Whereas nuclear introns are removed by a large ribonucleoprotein complex called the spliceosome, each bacterial intron encodes a distinct maturase protein facilitating its splicing. The evolutionary transition from the highly specific, maturase-driven splicing toward the complex transacting spliceosomal machinery is unclear. Without any "living fossils" with a protospliceosome, a key question is how the emerging intron diversification was accompanied by a splicing machinery with the ability to act on multiple targets. At least initially, the already existing splicing factors encoded by the invading group II introns, i.e., modified maturases, may have fulfilled this task.In present day organelles, maturase genes can be found in fungal and plant mitochondria as well as in plant chloroplasts. Although mitochondria are not directly amenable to transgenic studies, there are a number of plant species, foremost tobacco, for which directed mutagenesis of chloroplast genomes is possible (3). All land plant chloroplasts with the exception of some parasitic species in the genus Cuscuta (4, 5) contain a single maturase gene called matK in the intron of the lysine tRNA-K (UUU) gene. matK is expressed at least in green tissue (6-8). Sequence and structural homologies are found to the RNA binding motif (domain X) and the reverse transcriptase domain typically found in bacterial maturases (9, 10). Both domains are involved in splicing. By contrast, domains required for the mobility of group II introns are missing in MatK.The function of matK in chloroplasts is so far unknown, but its position inside the trnK gene suggests a role for splicing the trnK precursor. Importantly, it has been suggested that other chloroplast introns are targeted by MatK as well: firstly, several chloroplast genomes o...

A short PPR protein required for the splicing of specific group II introns in angiosperm chloroplasts

“…THA8 is required for ycf3-2 splicing and strongly enhances trnA splicing. THA8 promotes trnA splicing in conjunction with the nucleus-encoded proteins WTF1 and RNC1, and, most likely, with the chloroplast gene product MatK (Zoschke et al 2010). THA8 is also the third nucleus-encoded splicing factor discovered for ycf3-2, following OTP51 (Falcon de Longevialle et al 2008) and APO1, a member of a plant-specific protein family with a novel zinc-dependent RNA-binding domain (Watkins et al 2011).…”

“…S2B). The MatK protein encoded in the trnK intron associates with all chloroplast subgroup IIA introns and is likely to be required for their splicing (Zoschke et al 2010). This information is summarized (Jenkins et al 1997;Vogel et al 1999;Jenkins and Barkan 2001;Till et al 2001;Ostheimer et al 2003;Barkan 2006, 2007;Schmitz-Linneweber et al 2006;Watkins et al 2007;Asakura et al 2008;Beick et al 2008;Falcon de Longevialle et al 2008;Prikryl et al 2008;Kroeger et al 2009;Chateigner-Boutin et al 2011;Watkins et al 2011).…”

APO1 Promotes the Splicing of Chloroplast Group II Introns and Harbors a Plant-Specific Zinc-Dependent RNA Binding Domain

“…Experimental data are lacking for the Arabidopsis orthologs of CRS2, PPR4, WTF1, RNC1, and PPR5, so the set of splicing factors shown for introns found in Arabidopsis but not maize (clpP-1, clpP-2, and rpoC1) is likely to be incomplete. The chloroplast-encoded protein MatK associates in vivo with all of the subgroup IIA introns except for clpP-int2 (Zoschke et al, 2010) and is anticipated to be required for the splicing of its intron ligands. The ndhA and ndhB introns are not designated as APO1 ligands because the splicing defects in apo1 mutants were mild.…”

Post-transcriptional Regulation in Mitochondria of Rice and Wheat at Low Temperatures

“…This indicates that an additional component, such as a helper protein, is required for splicing; however, none of the angiosperm organellar introns encodes an intronic ORF for splicing, except two (MAT-R encoded within nad1 intron 4 in mitochondria and MatK encoded within trnK in chloroplasts), and no such ORF is encoded alone in the organellar genome 6 . Recently, a report suggested that MatK associates with some group II introns in tobacco chloroplasts 83 . Nevertheless, nucleus-encoded machinery must be imported into both mitochondria and chloroplasts to help splicing.…”