The E-coli RhIE RNA helicase regulates the function of related RNA helicases during ribosome assembly

Escherichia coli contains five members of the DEAD-box RNA helicase family, a ubiquitous class of proteins characterized by their ability to unwind RNA duplexes. Although four of these proteins have been implicated in RNA turnover or ribosome biogenesis, no cellular function for the RhIE DEAD-box protein has been described as yet. During an analysis of the cold-sensitive growth defect of a strain lacking the DeaD/CsdA RNA helicase, rhIE plasmids were identified from a chromosomal library as multicopy suppressors of the growth defect. Remarkably, when tested for allele specificity, RhIE overproduction was found to exacerbate the cold-sensitive growth defect of a strain that lacks the SrmB RNA helicase. Moreover, the absence of RhIE exacerbated or alleviated the cold-sensitive defect of deaD or srmB strains, respectively. Primer extension and ribosome analysis indicated that RhIE regulates the accumulation of immature ribosomal RNA or ribosome precursors when deaD or srmB strains are grown at low temperatures. By using an epitope-tagged version of RhIE, the majority of RhIE in cell extracts was found to cosediment with ribosome-containing fractions. Since both DeaD and SrmB have been recently shown to function in ribosome assembly, these findings suggests that WE genetically interacts with srmB and deaD to modulate their function during ribosome maturation. On the basis of the available evidence, I propose that RhIE is a novel ribosome assembly factor, which plays a role in the interconversion of ribosomal RNA-folding intermediates that are further processed by DeaD or SrmB during ribosome maturation.