GLOBAL REGULATION BY THE SMALL RNA-BINDING PROTEIN CSRA AND THE NONCODING RNA MOLECULE CSRB

Csr (carbon storage regulator) is a recently discovered global regulat ory system that controls bacterial gene expression post-transcriptiona lly. Its effector is a small RNA-binding protein referred to as CsrA o r, in phytopathogenic Erwinia species, RsmA (repressor of stationary p hase metabolites), Numerous genes whose expression occurs in the stati onary phase of growth are repressed by csrA/rsmA, and csrA activates c ertain exponential-phase metabolic pathways. Glycogen synthesis and ca tabolism, gluconeogenesis, glycolysis, motility, cell surface properti es and adherence are modulated by csrA in Escherichia coil, while the production of several secreted virulence factors, the plant hypersensi tive response elicitor HrpN(Ecc) and, potentially, other secondary met abolites are regulated by rsmA in Erwinia carotovora. CsrA represses g lycogen synthesis by binding to and destabilizing glgCAP mRNA and is h ypothesized to repress other genes by a similar mechanism, The second component of the Csr system is CsrB (AepH in Erwinia species), a nonco ding RNA molecule that forms a large globular ribonucleoprotein comple x with approximately 18 CsrA subunits and antagonizes the effects of C srA in vivo, Highly repeated sequence elements found within the loops of predicted stem-loops and other single-stranded segments of CsrB RNA may facilitate CsrA binding. Current information supports a model in which CsrA exists in an equilibrium between CsrB and CsrA-regulated mR NAs, which predicts that CsrB levels may be a key determinant of CsrA activity in the cell, The presence of csrA homologues in phylogenetica lly diverse species further suggests that this novel kind of regulator y system is likely to play a broad role in modulating eubacterial gene expression.