Guanosine tetraphosphate (ppGpp) is a good candidate for a stable RNA synthesis regulator at different growth rates because: high ppGpp levels present during the stringent response are known to inhibit stable RNA synthesis, and there is an inverse correlation between ppGpp concentration and stable RNA synthesis at different growth rates. A system was constructed allowing the user to control intracellular ppGpp levels within the range characteristic of exponentially growing strains, while not allowing cells to adjust ppGpp in response to environmental changes. E. coli strains were transformed with a plasmid containing a truncated relA allele to provide ppGpp. Results show that ppGpp is a major growth rate effector during exponential growth in bacteria
