Fig. 5.

Working model of AtPRMT3 in pre-rRNA processing. Two pre-rRNA processing pathways, which are different in cleavage order but not cleavage site, coexist in Arabidopsis. The P-A3 fragment and 32S rRNA are diagnostic for the major and minor pathways, respectively. AtPRMT3 in vivo promotes the major pathway and inhibits the minor pathway, probably by promoting the processing kinetics of site A3 in ITS1. Disruption of AtPRMT3 inhibits the A3 cleavage activity, causes unbalanced pre-rRNA processing pathways with down-regulation of the major pathway (P-A3) and up-regulation of the minor pathway (32S rRNA), and displays pre-rRNA processing defects as well as developmental phenotypes shared by ribosome biogenesis mutants.