FIG 10.

Model for CsrA-induced Rho-dependent attenuation control of pgaABCD expression in E. coli. The upstream region of the pgaA leader transcript folds into an imperfect hairpin that includes two potential CsrA binding sites in the upstream segment of the stem. The downstream segment of the stem includes a large part of a rut site, and its inclusion in the secondary structure prevents Rho binding and allows transcription to read through into the structural genes of the pgaABCD operon. In the presence of active CsrA, a dimeric form of the protein binds both CsrA binding sites simultaneously and unfolds the leader hairpin to expose the entire rut site. This configuration allows Rho to bind to the rut site and cause transcription termination within the leader region.