Escherichia coli rnhA mutants devoid of RNase HI exhibit constitutive stable DNA 
replication, cSDR, which is thought to be initiated from R-loops stabilized in 
the absence of RNase HI. We found that a combination of an rnhA and a recG 
mutation is lethal to the cell. recG mutations that inactivate the helicase 
activity of RecG protein and inhibit reverse branch migration of Holliday 
junctions impart phenotypes resembling those of rnhA mutants. Thus, recG mutants 
display cSDR activity, and recG polA double mutants are inviable as are rnhA 
polA double mutants. These results suggest that the RecG helicase has a role in 
preventing R-loop formation. A model that R-loops are formed by assimilation of 
RNA transcripts into the duplex DNA is discussed. The model further postulates 
that RecA protein catalyzes this assimilation reaction and that RecG protein 
counteracts RecA in this reaction, resolves R-loops by its helicase activity, or 
does both.