Multiple activities of RNA-binding proteins S1 and Hfq

In all organisms, RNA-binding proteins participate in modulating all the steps in the life cycle of RNA, including transcription, folding, translation and turnover. In bacteria, RNA-binding proteins may be specific for a few RNA targets (e.g., several ribosomal proteins that recognize both rRNA during ribosome assembly and their own mRNAs when acting as highly specific autogenous repressors) or function as global regulators implicated in numerous regulatory networks. Some RNA-binding proteins combine all these features, and this particularly concerns the ribosomal protein S1 and the Sm-like protein Hfq. S1 is a key mRNA-binding protein in gram-negative bacteria; it recognizes mRNA leaders and provides binding of diverse mRNAs to the ribosome at the initiation step of translation. Moreover, S1 is a highly specific autogenous repressor that is able to distinguish its own mRNA from all the others. Hfq is recognized as a global regulator that facilitates small RNA-mRNA interactions in bacteria; it thereby controls the expression of many mRNAs either positively or negatively. In addition, these two proteins were reported to affect transcription, RNA degradation and other processes. Although they have no sequence specificity, Hfq and S1 preferentially bind A/U-rich single-stranded RNA regions; despite this, they nevertheless carry out very different tasks in the cell. This review is focused on the diversity of functions that can be performed by these abundant RNA-binding bacterial proteins.