Transcription terminator involved in the expression of the int gene of phage lambda

The phage lambda int gene is transcribed from two different promoters, pI and pL. Transcription from pI results in efficient synthesis of Int protein whereas transcription originating from pL results in poor int expression. The differential expression of Int from these two transcripts is dependent upon a site (sib) located distal to the int gene [Guarneros and Galindo, Virology 95 (1979) 119-126; Guarneros et al., Proc. Natl. Acad. Sci. USA 79 (1982) 238-242]. We have examined pI-promoted transcription in the region beyond the int coding sequence. The int mRNA extends to a site designated tI, which is located 277 nucleotides beyond int. Characterization of transcription at tI indicates that tI terminates with 75% efficiency in vitro, and that its efficiency is over 95% in vivo. The region between int and tI contains the regulatory signals needed for phage lamba integration and appears to be untranslated. The termination site overlaps with the sib control region that reduces Int synthesis from pL transcripts.