Repression of transcription from the b2-att region of coliphage lambda by integration host factor

The central b2-att region of coliphage lambda is known to be transcriptionally active in vitro, but silent in vivo in lambda lysogens. To explain such in vivo repression of transcription originating in the b2-att region, we explored the effect of the Escherichia coli integration host factor (IHF), the product of E. coli genes himA and himD, especially since the att region contains several IHF-binding sites. Using various lambda DNA templates, we mapped the transcripts which are initiated in vitro in the attP region by the RNA polymerase and found that there are three rightward (RI, RII, and RIII) and one leftward (LI) transcripts. All four of them are repressed by a factor of about 10 by 10 micrograms IHF/ml. Moreover, in in vivo experiments we found that plasmids carrying the attP fragment cannot be established and maintained in IHF-hosts. These results indicate that IHF may play a significant auxiliary role in repressing transcription in the prophage state.