Transcription termination at lambda tR1 is mediated by interaction of rho with specific single-stranded domains near the 3' end of cro mRNA.

To determine whether E. coli rho protein mediates termination of transcription by interacting with specific segments of the nascent transcript, DNA oligonucleotides were used to sequester segments of phage lambda cro mRNA in hybrid helices. Formation of hybrids was demonstrated with RNAase H assays. Oligonucleotides complementary to either of two distinct, single-stranded sequences near the 3' end inhibited rho action at tR1, while oligonucleotides complementary to the sequence between those segments or to more 5' segments did not. The inhibitory oligonucleotides did not affect the elongation of cro mRNA or rho action on other transcripts. The results indicate that termination of transcription at tR1 is dependent upon contact of rho factor with specific, single-stranded domains near the 3' end of cro mRNA.