The Control of Ribonucleic Acid Synthesis in Escherichia coli

We have examined the functional relationship in vivo between purine ribonucleoside triphosphate pool size and the rate of the RNA accumulation. Our findings are as follows. The imposition of stringency leads to a roughly 40% drop in the GTP pool which occurs very quickly; it is complete in less than 10 min, and occurs simultaneously with, or just before, the shutoff of RNA accumulation. A somewhat slower 30% drop in the ATP pool was also observed. In a relaxed mutant, there is a momentary shrinkage of both pools, but they quickly return to normal and thereafter expand. The shrinkage of the ATP and GTP pools during stringency cannot be accounted for as a feedback response to blockade of RNA accumulation; when RNA accumulation is blocked directly by uracil starvation, the ATP and GTP pools expand rather than contract. Shrinkage of the purine ribonucleoside triphosphate pools may be sufficient to account for the reduced rate of RNA accumulation during stringency. When the synthesis of ATP and GTP is blocked directly by purine starvation, the rate of RNA accumulation falls drastically with only about a 40% shrinkage of the two pools. Thus net RNA synthesis in vivo is much more sensitive to purine ribonucleoside triphosphate concentration than is RNA polymerase activity in vitro. Shrinkage of the GTP pool alone may be sufficient to account for the response of RNA accumulation to stringency. When GTP synthesis is blocked directly by guanine or guanosine starvation, a moderate decrease in the GTP pool results in a disproportionate reduction in net RNA synthesis. Despite virtually complete inhibition of net RNA synthesis during guanosine starvation, protein synthesis proceeds at about 25% of the normal rate. This suggests that the synthesis and turnover of messenger RNA is much less affected by a moderate decrease in GTP than is the accumulation of stable forms of RNA.