Figure 5.

Model for translational activation of rpoS mRNA by DsrA, Hfq and CsdA. (A) At low temperature and during exponential growth RNase III cleavage occurs in the rpoS leader, which primes rapid decay of the mRNA.²⁵ As DsrA·rpoS annealing (Fig. 3) and consequently rpoS translation (Fig. 1) did not occur in the csdA- strain, intramolecular cleavage in the rpoS leader most likely accounts for the reduced steady state levels of rpoS mRNA in the latter when compared to the wild-type strain (see Fig. 3). (B) Hfq-DsrA binding to A-rich segments (A6 and/or AAN₍₄₎³⁰) in the rpoS leader occurs at low temperature when DsrA is expressed³⁰ (for clarity only one possible Hfq binding site is occupied by the Hfq-DsrA complex). CsdA aids in melting the iss and perhaps in unfolding of DsrA. (C) Annealing between DsrA and the rpoS mRNA occurs opposite of the rbs. (D) Translation of rpoS ensues and intermolecular RNase III cleavage of the DsrA·rpoS duplex (at G₋₁₁₂ in rpoS mRNA) prevents reuse of DsrA. Hfq is eventually recycled as a consequence of DsrA·rpoS base-pairing and/or by CsdA action. The segment of the iss is in red. Scissors signify RNase III cleavage. All other components are denoted.