Tandem transcription-termination sites in the late rightward operon of bacteriophage lambda.

A transcription termination site (designated as t'R2) is located between the rightward late t'R1 terminator and the S gene of phage lambda. This t'R2 terminator is rightward and absolutely dependent on the rho factor, being about 45% effective in rho+ E. coli and only 6% in rho- cells at 30 degrees C. This 7.5-fold rho dependence of t'R2 is in contrast to that of tR1 (4.5 fold, from about 81% to 18%) and the partial rho dependence of t'R1 (1.4 fold, from 96% to 67%). At the elevated temperature of 42 degrees C, t'R2 becomes 1.5 times more leaky (with about 2.5-fold reduction in termination efficiency) than tR1 or t'R1 (with only 1.1-fold reduction) in rho+ hosts. The calculated joint efficiencies of t'R1 and t'R2 are 98% in rho+ cells at 30 degrees C. t'R2 is also active in vitro, but only in the presence of rho factor, whereas t'R1 is active both in the presence and absence of rho. However, the in vitro termination at t'R1 is enhanced about 1.7-fold by the rho factor. The properly oriented lambda nutR site together with the N gene function bring about almost complete antitermination at t'R2 (96% effective), but incomplete at t'R1 (72%). The termination points at t'R2 are located around 532-534 bp to the right of the s'R startpoint of the p'R-initiated RNA on lambda DNA (or 338-340 bp downstream of t'R1) and 66-68 bp to the left of the S gene, as determined by S1 mapping. The t'R2 termination points are located within a dyad symmetry region which, in the transcript, is able to form a hairpin structure consisting of 16 bp in the stem and 6 bases in the loop. It is proposed that t'R2 acts as a second terminator to block any readthrough transcription initiated at the late promoter p'R into the late genes of phage lambda.