An eIF-4A-like protein is a suppressor of an Escherichia coli mutant defective in 50S ribosomal subunit assembly

Abstract

The assembly of ribosomes in bacterial cells is a complex process that remains poorly characterized. The in vitro assembly of active ribosomal su bun its from purified RNA and protein components indicates that all of the information for proper assembly resides in the primary sequences of these macromolecules. On the other hand, the in vitro requirement of unphysiological heating steps suggests that this pathway may not accurately reflect the in vivo pathway, and that other proteins may be required. One approach to identify any additional proteins is to isolate second-site rever-tants of mutants defective in ribosome assembly¹. Ribosomal pro-tein L24 is essential in the assembly of 508 su bun its^2–5. We have identified an Escherichia coli gene, srmB, that, when expressed at high copy number, can suppress the effect of a temperature-sensitive lethal mutation in L24. The SrmB amino-acid sequence has sequence identity with mouse translation initiation factor eIF-4A and with the human nuclear protein, p68. The purified SrmB protein is a nucleic acid-dependent ATPase, like eIF-4A, but can also bind RNA in the absence of ATP and other auxiliary protein factors. The RNA dependent ATPase activity of SrmB suggests that like, eIF-4A, it could be involved in specific alter-ations of RNA secondary structure.