Towards a structural framework for 40S assembly S Granneman et al conserved regions than over the eukaryotic-speciﬁc insertion elements. In Escherichia coli, the emphasis in research into ribosome synthesis has been on the analysis of in vitro reconstitution rather than in vivo assembly. However, binding sites have been characterized for some factors. The RNA-binding sites for the bacterial orthologue of Dim1 (KsgA) bound to the 30S ribosomal subunit were determined by in vitro directed hydroxyl radical cleavage and footprinting experiments (Xu et al, 2008). In E. coli, KsgA contacts rRNA regions in the 30S subunit that surrounds the modiﬁcation sites in H45 including H11, 24, 27, 28 and 44 (Supplementary Figure 5) and these interactions are proposed to be important in preventing premature interactions of pre-30S particles with 2032 The EMBO Journal VOL 29 | NO 12 | 2010 & 2010 European Molecular Biology Organization