Figure 1.

(A) λcI binds cooperatively to operator sites O_R1 and O_R2 to activate transcription from P_RM. Activation is mediated by the λcI dimer bound at O_R2, which likely contacts the σ⁷⁰ subunit of RNAP. (B) Genetic strategy for detecting the interaction between λcI and region 4 of σ⁷⁰. Replacement of the RNAP αCTD with region 4 of σ⁷⁰ permits interaction between the transplanted region of σ⁷⁰ and a λcI dimer bound adjacent to an auxiliary −35 element. The artificial promoter derivative plac O_R2–55/Cons-35 is shown; this bears the auxiliary −35 element (TTGACA) and the λ operator O_R2, centered 45.5 bp and 55 bp, respectively, upstream of the transcriptional start site of the lac promoter.