Translation of two nested genes in bacteriophage P4 controls immunity-specific transcription termination

In phage P4, transcription of the left operon may occur from both the constitutive P-LE promoter and the regulated P-LL promoter, about 400 nucleotides upstream of P-LE. A strong Rho-dependent termination site, t(imm), is located downstream of both promoters. When P4 immunity is expressed, transcription starting at P-LE is efficiently terminated at t(imm), whereas transcription from P-LL is immunity insensitive and reads through t(imm). We report the identification of two nested genes, kil and eta, located in the P4 left operon. The P4 kil gene, which encodes a 65-amino-acid polypeptide, is the first translated gene downstream of the P-LE promoter, and its expression is controlled by P4 immunity. Overexpression of kil causes cell killing. This gene is the terminal part of a longer open reading frame, eta, which begins upstream of P-LE. The eta gene is expressed when transcription starts from the P-LL promoter. Three likely start codons predict a size between 197 and 199 amino acids for the Eta gene product. Both kil and eta overlap the t(imm) site. By cloning kil upstream of a tRNA reporter gene, we demonstrated that translation of the kil region prevents premature transcription termination at t(imm). This suggests that P4 immunity might negatively control kil translation, thus enabling transcription termination at t(imm).(.) Transcription starting from P-LL proceeds through t(imm). Mutations that create nonsense codons in eta caused premature termination of transcription starting from P-LL. Suppression of the nonsense mutation restored transcription readthrough at t(imm). Thus, termination of transcription from P-LL is prevented by translation of eta.