THE RNA MOLECULE CSRB BINDS TO THE GLOBAL REGULATORY PROTEIN CSRA ANDANTAGONIZES ITS ACTIVITY IN ESCHERICHIA-COLI

The RNA-binding protein CsrA (carbon storage regulator) is a new kind of global regulator, which facilitates specific mRNA decay, A recombin ant CsrA protein containing a metal-binding affinity tag (CsrA-H6) was purified to homogeneity and authenticated by N-terminal sequencing, m atrix-assisted laser desorption/ionization time of flight mass spectro metry, and other studies. This protein was entirely contained within a globular complex of approximately 18 CsrA-H6 subunits and a single si milar to 350-nucleotide RNA, CsrB. cDNA cloning and nucleotide sequenc ing revealed that the csrB gene is located downstream from syd in the 64-min region of the Escherichia coli K-12 genome and contains no open reading frames. The purified CsrA-CsrB ribonucleoprotein complex was active in regulating glg (glycogen biosynthesis) gene expression ill v itro, as was the RNA-free form of the CsrA protein. Overexpression of csrB enhanced glycogen accumulation in E. coti, a stationary phase pro cess that is repressed by CsrA, Thus, CsrB RNA is a second component o f the Csr system, which binds to CsrA and antagonizes its effects on g ene expression, A model for regulatory inter actions in Csr is present ed, which also explains previous observations on the homologous system in Erwinia carotovora. A highly repeated nucleotide sequence located within predicted stem-loops and other single-stranded regions of CsrB, CAGGA(U/A/C)G, is a plausible CsrA-binding element.