Nature 444, 503–506 (2006); doi:10.1038/nature05182 (published online 29 October 2006)

During the preparation of the manuscript, we inadvertently mislabelled ribosomal protein L32 as ribosomal protein L18 when interpreting the density based on the 50S coordinates (PDB accession number 2AW4). Therefore, whenever L18 is mentioned in the text and in Figs 3 and 4, it should be considered to refer to ribosomal protein L32. Our results and conclusions are not affected.