Regulation of leftward transcription in the J-b2-att region of coliphage lambda

Summary

The leftward transcription of the central region of the mature coliphage λ genome, which extends from gene J to the attachment point att and which includes the b2 region, was studied. Upon thermal induction of λc _{I, 857} lysogens, the rate of the leftward transcription of the central region reaches a sharp maximum (1–2% of the total, i.e., λ + Escherichia coli, transcription) at about 5 min at 42° C, and then rapidly declines. This rapid decline was not observed in induced λ ⁻_x (p _R ⁻; phenotypically tof ⁻) lysogens. Upon induction of a lysogen carrying a single, nonexcisable int ⁻ prophage, the l strand of the central region is not transcribed, whereas in an induced int ⁺ or a double int ⁻ or int ⁺ lysogen the l strand of the central region is transcribed both early and late in λ development. It is concluded that the leftward transcription of the central region is not autonomously initiated, but depends on the coupling between b2 and the att-N region containing the autonomous P _L promoter. The leftward transcription of the central region is not restricted to b2, but can also proceed to the left of it. The results confirm the previous conclusion, based on a study of the late rightward transcription of the b2 segment (Bøvre and Szybalski, 1969), that convergent transcriptions overlap in the J-b2-att region, which comprises about 20% of the λ genome, leading to the synthesis of mutually complementary RNA sequences.