Cryo‐electron microscopy visualization of a large insertion in the 5S ribosomal RNA of the extremely halophilic archaeon Halococcus morrhuae

The extreme halophile Halococcus morrhuae (ATCC(®) 17082) contains a 108‐nucleotide insertion in its 5S rRNA. Large rRNA expansions in Archaea are rare. This one almost doubles the length of the 5S rRNA. In order to understand how such an insertion is accommodated in the ribosome, we obtained a cryo‐electron microscopy reconstruction of the native large subunit at subnanometer resolution. The insertion site forms a four‐way junction that fully preserves the canonical 5S rRNA structure. Moving away from the junction site, the inserted region is conformationally flexible and does not pack tightly against the large subunit. The high‐salt requirement of the H. morrhuae ribosomes for their stability conflicted with the low‐salt threshold for cryo‐electron microscopy procedures. Despite this obstacle, this is the first cryo‐electron microscopy map of Halococcus ribosomes.