Plant-specific ribosome biogenesis factors in Arabidopsis thaliana with essential function in rRNA processing.

rRNA processing and assembly of ribosomal proteins during maturation of ribosomes involve many ribosome biogenesis factors (RBFs). Recent studies identified differences in the set of RBFs in humans and yeast, and the existence of plant-specific RBFs has been proposed as well. To identify such plant-specific RBFs, we characterized T-DNA insertion mutants of 15 Arabidopsis thaliana genes encoding nuclear proteins with nucleotide binding properties that are not orthologues to yeast or human RBFs. Mutants of nine genes show an altered rRNA processing ranging from inhibition of initial 35S pre-rRNA cleavage to final maturation events like the 6S pre-rRNA processing. These phenotypes led to their annotation as 'involved in rRNA processing' - IRP. The irp mutants are either lethal or show developmental and stress related phenotypes. We identified IRPs for maturation of the plant-specific precursor 5'-5.8S and one affecting the pathway with ITS2 first cleavage of the 35S pre-rRNA transcript. Moreover, we realized that 5'-5.8S processing is essential, while a mutant causing 6S accumulation shows only a weak phenotype. Thus, we demonstrate the importance of the maturation of the plant-specific precursor 5'-5.8S for plant development as well as the occurrence of an ITS2 first cleavage pathway in fast dividing tissues.