During exponential growth conditions, bacteria constitutively synthesize ribosomes at a high rate to enable rapid protein synthesis and cell division. When environmental conditions can no longer support exponential growth, the "stringent response" rapidly decreases the level of ribosome synthesis.
When an uncharged tRNA binds to the A site of a ribosome, indicating a lack of amino acids available for protein synthesis, the ribosome synthesizes guanosine tetraphosphate (ppGpp). This signal molecule interacts with RNA polymerase to decrease transcription of genes that encode ribosomal RNA (rRNA). Reduced levels of rRNA decrease the number of functional ribosomes that can be made but do not directly impact the production of ribosomal proteins.
To prevent the production of unneeded ribosomal proteins, a second level of regulation occurs. When not bound to rRNA (as part of a ribosomal subunit), some ribosomal proteins will bind to mRNA molecules that encode these proteins. Binding of the mRNA by a ribosomal protein prevents translation of the mRNA and reduces the number of ribosomal proteins being synthesized by the cell.
How might the stringent response be beneficial to a bacterial cell?