A key translational regulator that binds mRNA to regulate translation initiation and/or mRNA stability. Mediates global changes in gene expression, shifting from rapid growth to stress survival by linking envelope stress, the stringent response and the catabolite repression systems. Usually binds in the 5'-UTR; binding at or near the Shine-Dalgarno sequence prevents ribosome-binding, repressing translation, binding elsewhere in the 5'-UTR can activate translation and/or stabilize the mRNA. Its function is antagonized by small RNA(s).