Ribonucleases (RNases) are important factors for the establishment of virulence in an increasing number of pathogens. These enzymes are responsible for the maturation and degradation of RNA molecules, being key players in RNA metabolism. In this opinion we will focus on exoribonucleases from the RNB family, whose members are known to be involved in the virulence of several microorganisms. Members of this family are present in all domains of life. They hydrolyze RNA in the 3′–5′ direction in a processive way (reviewed in Matos et al., 2011). RNase II from Escherichia coli is the prototype of this family, which also includes bacterial RNase R and eukaryotic Rrp44/Dis3 proteins. These proteins present a similar modular organization with cold shock domains (CSD) at the N-terminal region involved in RNA binding, a central RNB domain responsible for catalysis, and a Cterminal S1 domain, also involved in RNA binding (Amblar et al., 2006; Frazão et al., 2006). The RNB domain (characteristic from this family of enzymes) is very wellconserved, and the residues involved in catalysis are found in all members of the family (Barbas et al., 2008). These results point toward a conservation of the mechanism of action, as shown by the characterization of several enzymes of this family (Matos et al., 2009; Reis et al., 2013a). Exoribonucleases from the RNB-family have a wide spectrum of functions in the cell. In eukaryotes Rrp44/Dis3 and Dis3L1 are the catalytic subunit of the exosome, a crucial RNA degradation complex (reviewed in Arraiano et al., 2013). Dis3L2 was recently discovered and defines a novel eukaryotic RNA degradation pathway (Malecki et al., 2013). In prokaryotes, they are often essential for growth and viability and can be developmentally regulated. They are important for stress responses and are also involved in RNA and protein quality control (reviewed in Matos et al., 2011). We will discuss some of these functions, focusing on RNase R, which has been implicated in pathogenesis.
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