How to avoid undesirable interactions during ribosome assembly?

Ribosome subunit assembly in bacteria is assisted by several non-ribosomal proteins, the absence of which leads to assembly defects. The two DEAD-box RNA helicases SrmB and DeaD/CsdA are required for efficient assembly of the ribosome large subunit, in particular at low temperature, but their sites of action on rRNA were not known until now. In this issue of Molecular Microbiology, Proux et al. show that SrmB acts far away from its tethering site on the assembly intermediate particle. A genetic screen identified mutations in complementary sequences of 23S and 5S rRNA that help to bypass SrmB deficiency, partially correcting the large subunit assembly defect. The results suggest that 5S rRNA and 23S rRNA can interact via base-pairing, forming a non-native structure that needs to be corrected. The authors discuss attractive hypotheses on SrmB acts during large subunit assembly.