Molecular recognition of RhlB and RNase D in the Caulobacter crescentus RNA degradosome

The endoribonuclease RNase E is a key enzyme in RNA metabolism for many bacterial species. In Es-cherichia coli, RNase E contributes to the majority of RNA turnover and processing events, and the enzyme has been extensively characterized as the central component of the RNA degradosome assembly. A similar RNA degradosome assembly has been described in the alpha-proteobacterium Caulobacter cres-centus, with the interacting partners of RNase E identified as the Kreb's cycle enzyme aconitase, a DEAD-box RNA helicase RhlB and the exoribonuclease polynucleotide phosphorylase. Here we report that an additional degradosome component is the essential exoribonuclease RNase D, and its recognition site within RNase E is identified. We show that, unlike its E. coli counterpart, C. crescentus RhlB interacts directly with a segment of the N-terminal catalytic domain of RNase E. The crystal structure of a portion of C. crescentus RNase E encompassing the helicase-binding region is reported. This structure reveals that an inserted segment in the S1 domain adopts an alpha-helical conformation, despite being predicted to be natively unstructured. We discuss the implications of these findings for the organization and mechanisms of the RNA degradosome.