Transcription termination and processing sites in the bacteriophage λ p_L operon

S₁ nuclease mapping was performed on transcripts from the major leftward operon of the bacteriophage λ in order to locate the 3′ ends of stable RNA species produced in vivo. The analysis was carried out on RNA purified from either an induced λ prophage or bacteria carrying a plasmid containing a large segment of λ including the intact θ_L operon through the bet gene. The S₁ nuclease mapping was performed on transcripts produced in the presence and the absence of the N antitermination function, and in the presence and the absence of either the RNase III processing enzyme or the Rho factor. The results of this work indicate that the intercistronic region between the N and ral genes of λ contains three sites at which transcripts end under N^-Rho⁺ conditions (positions on the λ sequence: 34,826, 34,558 and 34,393). The distal two correspond to the two sites previously described in this region as t_L1 (on both sides of the BamHI site). In the region between ral and Ea10, we mapped the 3′ ends of three species of RNA. The 3′ end of one species was found to be located 90 nucleotides proximal to t_L2a, at 34,000 in the λ sequence. The terminator at this site may be partially N-resistant. In an RNase III deficient host, an additional RNA species is formed. The 3′ end of this RNA species is located at t_L2a (33,910 on the λ sequence). In the presence of the antitermination N gene product, the readthrough transcripts are processed to form a 3′ end at position 33,980 on the λ sequence. These results suggest that elongation of transcription of the λ p_L operon is reduced gradually by clusters of termination located between genes and that the expression of the terminated products is further controlled by processing of the mRNA.