Mechanistic studies on the impact of transcription on sequence-specific termination of DNA replication and vice versa

Since DNA replication and transcription often temporally and spatially overlap each other, the impact of one process on the other is of considerable interest. We have reported previously that transcription is impeded at the replication termini of Escherichia coli and Bacillus subtilis in a polar mode and that, when transcription is allowed to invade a replication terminus from the permissive direction, arrest of replication fork at the terminus is abrogated. In the present report, we have addressed four significant questions pertaining to the mechanism of transcription impedance by the replication terminator proteins. Is transcription arrested at the replication terminus or does RNA polymerase dissociate from the DNA causing authentic transcription termination? How does transcription cause abrogation of replication fork arrest at the terminus? Are the points of arrest of the replication fork and transcription the same or are these different? Are eukaryotic RNA polymerases also arrested at prokaryotic replication termini? Our results show that replication terminator proteins of E. coli and B. subtilis arrest but do not terminate transcription. Passage of an RNA transcript through the replication terminus causes the dissociation of the terminator protein from the terminus DNA, thus causing abrogation of replication fork arrest. DNA and RNA chain elongation are arrested at different locations on the terminator sites. Finally, although bacterial replication terminator proteins blocked yeast RNA polymerases in a polar fashion, a yeast transcription terminator protein (Reb1p) was unable to block T7 RNA polymerase and E. coli DnaB helicase.