Ribosome biogenesis involves a large ensemble of trans-acting factors, which catalyse rRNA processing, ribosomal protein association and ribosomal subunit assembly. The circularly permuted GTPase Lsg1 is such a ribosome biogenesis factor, which is involved in maturation of the pre-60S ribosomal subunit in yeast. We identified two orthologues of Lsg1 in Arabidopsis thaliana. Both proteins differ in their C-terminus, which is highly charged in atLSG1-2 but missing in atLSG1-1. This C-terminus of atLSG1-2 contains a functional nuclear localization signal in a part of the protein that also targets atLSG1-2 to the nucleolus. Furthermore, only atLSG1-2 is physically associated with ribosomes suggesting its function in ribosome biogenesis. Homozygous T-DNA insertion lines are viable for both LSG1 orthologues. In plants lacking atLSG1-2 18S rRNA precursors accumulate and a 20S pre-rRNA is detected, while the amount of pre-rRNAs that lead to the 25S and 5.8S rRNA is not changed. Thus, our results suggest that pre-60S subunit maturation is important for the final steps of pre-40S maturation in plants. In addition, the lsg1-2 mutants show severe developmental defects, including triple cotyledons and upward curled leaves, which link ribosome biogenesis to early plant and leaf development.