Escherichia coli RNase R has dual activities, helicase and RNase

In Escherichia coli, the cold shock response occurs when there is a temperature downshift from 37 degrees C to 15 degrees C, and this response is characterized by induction of several cold shock proteins, including the DEAD-box helicase CsdA, during the acclimation phase. CsdA is involved in a variety of cellular processes. Our previous studies showed that the helicase activity of CsdA is critical for its function in cold shock acclimation of cells and that the only proteins that were able to complement its function were another helicase, RhlE, an RNA chaperone, CspA, and a cold-inducible exoribonuclease, RNase R. Interestingly, other major 3'-to-5' processing exoribonucleases of E. coli, such as polynucleotide phosphorylase and RNase II, cannot complement the cold shock function of CsdA. Here we carried out a domain analysis of RNase R and showed that this protein has two distinct activities, RNase and helicase, which are independent of each other and are due to different domains. Mutant RNase R proteins that lack the RNase activity but exhibit the helicase activity were able to complement the cold shock function of CsdA, suggesting that only the helicase activity of RNase R is essential for complementation of the cold shock function of CsdA. We also observed that in vivo deletion of the two cold shock domains resulted in a loss of the ability of RNase R to complement the cold shock function of CsdA. We further demonstrated that RNase R exhibits helicase activity in vitro independent of its RNase activity. Our results shed light on the unique properties of RNase R and how it is distinct from other exoribonucleases in E. coli.