Effect of altered efficiency of the RNA I and RNA II promoters on in vivo replication of ColE1-like plasmids in Escherichia coli.

Thermal inactivation of the dnaA gene product leads to a considerable decrease in the rate of replication of ColE1-like plasmids. To test the possibility that the dnaA protein may affect synthesis of RNA I, which is an inhibitor of primer formation, or synthesis of RNA II, which is the primer precursor for replication of ColE1 ( Tomizawa and Itoh 1982), the effect of the dnaA46 mutation on the efficiency of the RNA I and the RNA II promoters was examined. It appears that thermal inactivation of the dnaA protein results in a considerable increase in the activity of the RNA I promoter. We suggest that overproduction of RNA I in dnaA mutants grown at the restrictive temperature is responsible for the reduced replication of ColE1-like plasmids. It has been found that addition of rifampicin to cultures of the dnaA46 or the dna+ strain grown at 42 degrees C results in a dramatic increase in the rate of replication of ColE1-like plasmids. We show that the activity of the RNA II promoter at 42 degrees C is exceptionally resistant to rifampicin. In the presence of the drug, this leads, to an altered ratio of RNA I to RNA II, in favor of the latter RNA species.