Distinct 18S rRNA precursors are targets of the exosome complex, the exoribonuclease RRP6L2 and the terminal nucleotidyltransferase TRL in Arabidopsis thaliana.

The biosynthesis of ribosomal RNA and its incorporation into functional ribosomes is an essential and intricate process that includes production of mature ribosomal RNA from large precursors. Here, we analyse the contribution of the plant exosome and its co-factors to processing and degradation of 18S pre-RNAs in Arabidopsis thaliana. Our data show that, unlike in yeast and humans, an RRP6 homologue, the nucleolar exoribonuclease RRP6L2, and the exosome complex, together with RRP44, function in two distinct steps of pre-18S rRNA processing or degradation in Arabidopsis.

Polyadenylation-assisted RNA degradation processes in plants.

Polyadenylation is a multifunctional post-transcriptional modification that is best known for stabilizing eukaryotic mRNAs and promoting their translation. However, the primordial role of polyadenylation is to target RNAs for degradation by 3' to 5' exoribonucleases. Polyadenylation-assisted RNA degradation contributes to post-transcriptional control in the three genetic compartments of a plant cell: the nucleus, the chloroplast and the mitochondrion.

Coping with cryptic and defective transcripts in plant mitochondria.

Plant mitochondria are particularly prone to the production of both defective and cryptic transcripts as a result of the complex organisation and mode of expression of their genome. Cryptic transcripts are generated from intergenic regions due to a relaxed control of transcription. Certain intergenic regions are transcribed at higher rates than genuine genes and therefore, cryptic transcripts are abundantly produced in plant mitochondria.

Degradation of a polyadenylated rRNA maturation by-product involves one of the three RRP6-like proteins in Arabidopsis thaliana.

Yeast Rrp6p and its human counterpart, PM/Scl100, are exosome-associated proteins involved in the degradation of aberrant transcripts and processing of precursors to stable RNAs, such as the 5.8S rRNA, snRNAs, and snoRNAs. The activity of yeast Rrp6p is stimulated by the polyadenylation of its RNA substrates.

Arabidopsis GCP2 and GCP3 are part of a soluble gamma-tubulin complex and have nuclear envelope targeting domains.

In higher plants, microtubules (MTs) are assembled in distinctive arrays in the absence of a defined organizing center. Three MT nucleation sites have been described: the nuclear surface, the cell cortex and cortical MT branch points. The Arabidopsis thaliana (At) genome contains putative orthologues encoding all the components of characterized mammalian nucleation complexes: gamma-tubulin and gamma-tubulin complex proteins GCP2 to GCP6.

The plant Spc98p homologue colocalizes with gamma-tubulin at microtubule nucleation sites and is required for microtubule nucleation.

The molecular basis of microtubule nucleation is still not known in higher plant cells. This process is better understood in yeast and animals cells. In the yeast spindle pole body and the centrosome in animal cells, gamma-tubulin small complexes and gamma-tubulin ring complexes, respectively, nucleate all microtubules.