Mutations of bacteriophage lambda that define independent but overlapping RNA processing and transcription termination sites

Bacteriophage λ int gene expression is regulated differentially from transcripts originated at the p L and p I promoters. Transcripts initiated at p I terminate at the site t I and express int gene product efficiently. Polymerases starting at p L do not terminate at t I, due to the antiterminating activity of λ N protein. The p L transcripts are unable to express Int protein efficiently because sib, a control site overlapping t I in the unterminated RNA, is processed by host RNase III. We have isolated λsib − mutants by their inability to inhibit int expression from p L transcripts. sib mutations were genetically mapped to the left of the λ attachment site, and do not structurally alter this site for recombination. Several sib mutations do alter the nucleotide sequence of the overlapping sib and t I sites. The λsib − mutants tested prevent RNA processing but do not affect transcription termination in vivo.