Termination of the Escherichia coli asnC transcript. The DnaA protein/dnaA box complex blocks transcribing RNA polymerase.

Genes clockwise of oriC, the Escherichia coli replication origin (oriC-mioC-asnC), show anticlockwise transcription. The intergenic region between mioC and asnC contains both a terminator and a consensus DnaA-protein-binding site (dnaA box). We analysed termination in this region using galK expression to monitor for transcription. About 50% of the asnC transcripts were not terminated, and about 25% terminated at the asnC terminator. We found that the DnaA protein/dnaA box complex acts as a terminator of transcription for about 25% of the transcripts. Its efficiency could be increased by raising the level of DnaA protein, or it could be inactivated by deletion in the dnaA box or by thermal denaturation of the DnaA protein.