Regulation of r-protein operons through a translational feedback loop. In many r-protein operons, such as the one shown here, one of the r-proteins produced by the operon (in this case, protein L4) also functions as a translational repressor in a mechanism that involves sensing the relative levels of r-proteins and rRNAs. L4 protein has a higher affinity for rRNA and binds it preferentially over the r-protein mRNA. When levels of r-protein are low relative to rRNA, few L4 protein molecules are available to bind the mRNA and translation proceeds, making more r-proteins. But when r-protein concentrations build up in excess of rRNA levels, the L4 protein binds to the mRNA generated from the operon and blocks production of additional r-proteins by preventing efficient initiation of translation.