CONTROL OF GENE-EXPRESSION IN THE TEMPERATE COLIPHAGE-186 .8. CONTROL OF LYSIS AND LYSOGENY BY A TRANSCRIPTIONAL SWITCH INVOLVING FACE-TO-FACE PROMOTERS

The lysogenic and early lytic operons of the temperate coliphage 186 are transcribed divergently. Primer extension mapping of the 5′ ends of thesein vivo transcripts showed that the rightward lytic promoter,pR, and the leftward lysogenic promoter,pL, are arranged face-to-face, with their transcripts overlapping by 60 bases. We examined the control of transcription frompR andpL usinggalK as a reporter gene. The product of the lysogeniccI gene strongly repressedpR transcription while allowingpL transcription. The product of the lyticapl gene (formerlyCP75) strongly repressedpL transcription while allowingpR transcription. Thus, thecI-pR-pL-apl region functioned as a transcriptional switch, determining whether transcription was lytic or lysogenic. Also, thecI gene product was able to stimulatepL, possibly by alleviating an inhibition ofpL transcription caused by convergent transcription frompR. Other consequences of the face-to-face promoter arrangement are discussed. © 1990 Academic Press Limited.