Transcription terminates at lambda tR1 in three clusters.

Transcription of the rightward operon of bacteriophage lambda terminates in the presence of rho factor at a region known as tR1, located downstream of the structural gene for the lytic repressor cro. We demonstrate by nuclease Sl mapping that transcription termination at lambda tR1 occurs in vitro over a stretch of 60 nucleotides. End points of transcripts are clustered in three distinct regions, which we refer to as termination sites I, II, and III. Termination at site I is inhibited by L factor, whereas termination at sites II and III is not affected by L factor. The sensitivities of these sites to rho factor are in the order III greater than II greater than I. The cin-lcnc-l mutations abolish termination at site II but not at sites I and III; this result may explain the failure of these mutations to alleviate the phage requirement for N function for growth. Although possible stem-and-loop structures in the RNA transcript immediately upstream of each of the three termination sites can be found, no consistent correlation exists between the strengths of these stem-and-loop structures and the termination efficiencies of their respective sites.